Well, this is what I have already as a preliminary or near-finished set-up for a lower-than-standard-level-of-magic D&D campaign, with regards to spellcasting.
It's meant for use with the standard D&D classes but replacing the spellcasting classes with additional mundane classes, the generalist (stats are just like it sounds) and scion (more skill-and-feat-oriented than the generalist, less directly combat-capable).
However, these spellcasting rules would probably work fine with D20 Modern classes, with only one or two little tweaks. Add Spellcraft to the class skill list of the Smart Hero/Ordinary (and probably also the Dedicated and Charismatic Heroes/Ordinaries), or just put it in as part of an Occupation's benefits (though that wouldn't work for characters who choose to become spellcasters later rather than at 1st-level). Replace instances of Knowledge (arcana) in the following descriptions with D20 Modern's version which is Knowledge (arcane lore), or maybe it was Knowledge (occult lore) in D20M.......
I only have the guidelines and feats for duplicating wizardly spellcasting so far, but you can probably extrapolate easily enough from that to make equivalent feats and guidelines for bardic, clerical, druidic, paladinesque, rangeresque, and sorcerous spellcasting. Not quite sure what to suggest for those right now, though of course there should be slightly fewer feats required (and slightly different prerequisites) for the bardic spells progression, and slighter fewer still for the paladinesque and rangeresque spell progressions.
Also, I do not have any details in my notes yet for how to handle metamagic feats with this set-up, since it eliminates spell slots (the Spellcraft DC to cast more wizard spells that day just increases with each such spell already cast or fizzled). I do not have details yet on the minor differences for spellbooks and such in this setp-up, nor do I have details yet on how magerods and spellstaves (essentially alternatives to spellbooks) would work/be statted.
A note: since this stuff is meant for a somewhat-lower amount of magic prevalence (moreso with regards to magic items and magical monsters than with spellcasting), I also have a few more feats available for use with it; Apothecary gives some minor benefits regarding the use of certain skills; Chirurgeon allows more advanced uses of the Heal skill for surgery and such; Diverse gives extra skill points each level for use with a few non-actiony skills (Appraise, Craft, Knowledge, etc.); Great Versatility grants three bonus class skills of choice, but they have to be very specific (not 'all Craft skills' or the like). These things may not be relevant to your game, but if you are intending to go lower-magic with the D20 Modern classes in use, then it's something you may consider.
Wizards:
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A Knowledge [arcana] check is needed to learn a new wizard spell, and in order to learn a wizard spell, the character needs an Intelligence score of at least 10 + the spell's level, as well as at least 2 ranks of Knowledge [arcana] per spell level (1 rank for 0-level spells). To learn the spell is a check against DC 15 + the spell's level. Maximum number of wizard spells known is Intelligence modifier + the number of Knowledge [arcana] ranks possessed, but a spellbook, magerod, or spellstave allows twice that many to be known, if all are stored in the item.
Casting a wizard spell requires an Intelligence score of at least 10 + the spell's level, and also requires a Spellcraft check against a DC of 10 + double the spell's level. Failing the Spellcraft check by less than 5 points results in the spell fizzling after spending only a move-equivalent action in the attempted casting, but it still provokes attacks of opportunity as normal for casting a spell. If casting defensively, the Concentration check is made first, and no Spellcraft check is necessary if the Concentration check fails. Failing the Spellcraft check by 5 or more points results in spending the entire normal casting time before the spell fizzles, as well as the caster suffering 1 point of subdual damage plus an additional 1 point per spell level. If the caster is immune to subdual damage, they instead take normal damage.
Each wizard spell successfully cast, or each one that failed by 5+ points on the Spellcraft check, increases the Spellcraft DC for wizard spells for the rest of the day. This increase is +1 DC per 3 spell levels, rounded up. Cannot take 10 or take 20 on the Knowledge and Spellcraft checks for these purposes. The character can only cast wizard spells they have prepared that day. Only 1 spell per rank of Knowledge [arcana] can be prepared each day, plus a number of spells equal to the character's Intelligence modifier. Preparing a spell takes 10 minutes. A prepared spell can be replaced by preparing a different spell, but this takes an additional 10 minutes.
If the character uses a spellbook, magerod, or spellstave to know more spells than they could through memory alone, then they must have that item present and study it while preparing their spells, or else they cannot prepare any wizard spells. A prepared spell remains prepared until 24 hours have passed since its preparation, or until replaced by another prepared spell. Casting a spell does not remove it from memory. Only spells from the wizard spell list can be learned and cast as wizard spells. The wizard spell list is not considered to be your spell list unless you have the Wizard feat or the Specialist Wizard feat. If you have neither feat, then only the wizard spells you actually know are considered part of your spell list. The save DC for any saving throws against a wizard spell is equal to 10 + the spell's level + the caster's Intelligence modifier.
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Wizardly Spellcasting Feats:
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Arch Wizard (General)
Your wizardly spellcasting abilities are practically unsurpassed.
Prerequisites: Int 19+, Knowledge [arcana] 18+ ranks, Spellcraft 9+ ranks, Great Wizard feat, Journeyman Wizard feat, Master Wizard feat, Wizard feat or Specialist Wizard feat
Benefit: You now gain the capacity to learn and cast wizard spells of 9th-level, with the proper skills and ranks. You automatically learn 2 wizard spells of 9th-level, which counts toward your normal limits. Every time you learn a 9th-level wizard spell, you get 1 less skill point at that level (per 9th-level spell learned). Thus, with the 2 such spells granted by this feat, you get 2 fewer skill points at this level. This can reduce your number of skill points for the level to 0 but no lower. Separately from that, your base skill points per level are reduced by 1 with all classes and hit dice you have and may later gain, but this particular reduction will not reduce it to less than 1 skill point per level. Skill points lost by learning spells are factored in after this reduction. Lastly, upon gaining this feat, you henceforth gain a +2 bonus on all Knowledge [arcana] and Spellcraft checks.
Great Wizard (General)
Your wizardly spellcasting abilities have grown much stronger.
Prerequisites: Int 15+, Knowledge [arcana] 10+ ranks, Spellcraft 5+ ranks, Journeyman Wizard feat, Wizard feat or Specialist Wizard feat
Benefit: You now gain the capacity to learn and cast wizard spells of 5th-level and 6th-level, with the proper skills and ranks. You automatically learn 2 wizard spells of 5th-level, which counts toward your normal limits. Every time you learn a 5th-level or 6th-level wizard spell, you get 1 less skill point at that level (per 5th- or 6th-level spell learned). Thus, with the 2 such spells granted by this feat, you get 2 fewer skill points at this level. This can reduce your number of skill points for the level to 0 but no lower. Separately from that, your base skill points per level are reduced by 1 with all classes and hit dice you have and may later gain, but this particular reduction will not reduce it to less than 1 skill point per level. Skill points lost by learning spells are factored in after this reduction.
Journeyman Wizard (General)
Your wizardly spellcasting abilities have improved.
Prerequisites: Int 13+, Knowledge [arcana] 6+ ranks, Spellcraft 3+ ranks, Wizard feat or Specialist Wizard feat
Benefit: You now gain the capacity to learn and cast wizard spells of 3rd-level and 4th-level, with the proper skills and ranks. You automatically learn 2 wizard spells of 3rd-level, which counts toward your normal limits. Every time you learn a 3rd-level or 4th-level wizard spell, you get 1 less skill point at that level (per 3rd- or 4th-level spell learned). Thus, with the 2 such spells granted by this feat, you get 2 fewer skill points at this level. This can reduce your number of skill points for the level to 0 but no lower. Separately from that, your base skill points per level are reduced by 1 with all classes and hit dice you have and may later gain, but this particular reduction will not reduce it to less than 1 skill point per level. Skill points lost by learning spells are factored in after this reduction.
Master Wizard (General)
Your wizardly spellcasting abilities are incredibly powerful and advanced.
Prerequisites: Int 17+, Knowledge [arcana] 14+ ranks, Spellcraft 7+ ranks, Great Wizard feat, Journeyman Wizard feat, Wizard feat or Specialist Wizard feat
Benefit: You now gain the capacity to learn and cast wizard spells of 7th-level and 8th-level, with the proper skills and ranks. You automatically learn 2 wizard spells of 7th-level, which counts toward your normal limits. Every time you learn a 7th-level or 8th-level wizard spell, you get 1 less skill point at that level (per 7th- or 8th-level spell learned). Thus, with the 2 such spells granted by this feat, you get 2 fewer skill points at this level. This can reduce your number of skill points for the level to 0 but no lower. Separately from that, your base skill points per level are reduced by 1 with all classes and hit dice you have and may later gain, but this particular reduction will not reduce it to less than 1 skill point per level. Skill points lost by learning spells are factored in after this reduction.
Specialist Wizard (General)
You have learned to cast arcane magic spells, through study and rote formulas. However, your arcane studies have focused on only certain kinds of magic.
Prerequisites: Int 11+, Spellcraft 1+ ranks
Benefit: Knowledge [arcana] is a permanent class skill for you, at this level and all levels gained henceforth. You henceforth have the capacity to learn and cast wizard spells, but you are limited to learning and casting wizard spells of only 2nd-level and lower. At the level you gain this feat, you acquire 1 free rank in Knowledge [arcana] and automatically learn 2 wizard spells of 1st-level, which count towards your normal limits. You also learn 2 free wizard spells of 0-level, which do not count towards those limits. Every time you learn a 1st-level or 2nd-level wizard spell, you get 1 less skill point at that level (per 1st- or 2nd-level spell learned). Thus, with the 2 such spells granted by this feat, you get 2 fewer skill points at this level. This can reduce your number of skill points for the level to 0 but no lower.
However, upon taking this feat you must choose one primary school of magic for your wizard spells, and four secondary schools of magic for your wizard spells, while treating the other three schools of magic as banned to you, for purposes of wizard magic only. The eight schools of magic are Abjuration, Conjuration, Divination, Enchantment, Evocation, Illusion, Necromancy, and Transmutation. You lose the capacity to learn and cast wizard spells of the schools that are banned to you, and likewise lose the capacity to activate such spells in magic items (even versions of those spells that happen to be non-wizardly). You immediately lose all knowledge of how to cast and use such banned spells, if you already knew any such spells beforehand, but only if they are wizard spells. Thus, if they were previously part of your total wizard spells known, they are removed from that list. You are no longer considered to have those spells on your wizard spell list.
In exchange for your specialization, you gain certain added benefits from your deeper study of certain arcane forces. Your wizard spells of the primary school gain +2 to their saving throw DCs, and you gain +2 on caster level checks involving those wizard spells, as well as a +2 bonus on your own saving throws against wizard spells of that school. Lastly, you gain +2 on all Knowledge [arcana] and Spellcraft checks related to your primary school of magic, even regarding such effects of a non-wizardly nature, so long as they are connected to your primary school of magic. This makes it easier for you to learn and cast such spells, as a result.
Special: You cannot gain this feat if you possess the Wizard feat, and vice versa.
Wizard (General)
You have learned to cast arcane magic spells, through study and rote formulas.
Prerequisites: Int 11+, Spellcraft 1+ ranks
Benefit: Knowledge [arcana] is a permanent class skill for you, at this level and all levels gained henceforth. You henceforth have the capacity to learn and cast wizard spells, but you are limited to learning and casting wizard spells of only 2nd-level and lower. At the level you gain this feat, you acquire 1 free rank in Knowledge [arcana] and automatically learn 2 wizard spells of 1st-level, which count towards your normal limits; if your ranks in the skill are already maximized before gaining this feat, then you do not gain the free rank and these 1st-level spells known. Regardless, you also learn 2 free wizard spells of 0-level, which do not count towards those limits. Every time you learn a 1st-level or 2nd-level wizard spell, you get 1 less skill point at that level (per 1st- or 2nd-level spell learned). Thus, with the 2 such spells granted by this feat, you get 2 fewer skill points at this level. This can reduce your number of skill points for the level to 0 but no lower.
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