Responding to this very old thread just for a place to put my thoughts down ... This is how I would make different kinds of magic FEEL different within the mechanics of the game.
Arcane Magic:
No more spell slots for Wizards. Arcane magic by a wizard is a learned process. Instead, Arcana skill check to cast a spell with DC = (spell level + 1) x 2. So, DC 20 for 9th level spells down to DC 4 for 1st level spells. A nat 1 is not an automatic failure. To cast another spell requires you to wait that many rounds, so casting another 9th level spell will require you to wait 9 rounds even to cast a 1st level. You can cast a 1st level every round (b/c you would have waited 1 round). Cantrips and spells can be combined as with the existing rules b/c there is no waiting needed to cast cantrips beyond the existing rule. A skill check that doesn't succeed causes some kind of backlash based on the level so spell being attempted, with harshness also based on level. A skill check that succeeds, but only within 5 requires another roll of the same DC or gain a level of exhaustion (so, a 1st level spell might get a level of exhaustion on a roll of 9 and then a roll of 1-4, but that is with NO proficiency in Arcana). Learning spells takes time and tutelage. Time it takes to learn a spell is spell level squared in days in QUIET contemplation, study, and practice so probably not something that can be done while fighting monsters. So, 1 day to learn a first level spell, almost a month for a 5th (25 days), bit more than a month for a 6th (36 days) and nearly 3 months for a 9th. They then need to immediately demonstrate that they could cast the spell successfully (by passing the Arcana skill check for the spell), but this does not actually cast the spell. If they fail, that spell is not available to them because they couldn't figure out how it works, otherwise they now know the spell. Also, no more just getting two new spells when you level up, they have to be discovered on scrolls or spell books and then the learning can begin. (Maybe you can create your own spells but the learning time is tripled and costs mucho gold.) Also, with no more spell slots, there is no more upcasting. Instead the effect of upcasting is gained by using more expensive (read: you must adventure for them) spell components. Are they consumed? Maybe, case-by-case basis but I'd say generally yeah depending on how much upcasting power they confer and how good they are. This FEELS more like a wizard of fantasy to me.
To lower the impact of those rules: A spell can be held, up to the moment of casting, and then unleashed that round with the skill roll then taking place. Holding a spell allows the time needed to pass before casting another spell happens. This holding of a spell is an act of concentration and if concentration is broken the held spell is lost. So, a wizard could prepare a 2nd level spell, spend at least one round concentrating on it, and then unleash the spell (if a successful Arcana skill check of DC 6 is made), and then cast another spell on the following round. In this way a Wizard, with the right circumstances, planning, and luck cast two 9th level spells back-to-back and creates a high-risk/high-reward setup. Wizards are still limited to how high a level of spell they can cast as before (so no 9th level spells until achieving wizard level 17). Even with this planning, things could go wrong on the first (held) casting and cause a level of exhaustion, so this carries a level of risk. Wands could come in +1 to +3 varieties that provide a bonus to the Arcane casting check. How the enforced delay interacts with reaction spells like counterspell, feather fall, absorb elements, etc. would take some thinking. There might need to be an exception made for those but given that any cantrips or 1st level spells known can now be cast all day long every round, it's probably worth keeping the above rule as-is. I could also envision making the skill check harder when you know more spells (say, +1 to the DC for every 10 level of spells learned).
Divine Magic:
Spells slots still exist, but instead of subclasses gaining spells that are always considered prepared there are certain spells that are off limits (so, a god of darkness doesn't grant darkness spell, but denies light spells instead, for example). That's b/c 'spells' for clerics are actually miracles brought into the world by the power of their deity channeled through them. However, there is no rolling, they ALWAYS work b/c they ARE miracles. However, although guaranteed to work, they only work at the lowest level of possible success, meaning no more upcasting of cleric spells. Instead of preparing spells ahead of time, all spells are on the table at the moment it is the characters turn (as long as they have all the other requirements met such as material components), because spells slots don't represent a specific miracle/spell but a specific power level of miracle.
Sorcerous Magic:
This is arcane magic that comes from with, from 'the blood' itself because the caster is magic in some way. However, they don't need arcane focuses any more (why? They ARE magic). There is no rolling for success, because this isn't a skill. They still have spell slots and still the same limit on spell selection as before (once gained, they can't change; this is the downside of being a sorcerer). However, in order to upcast requires them to sacrifice 1 HD per level of increase, with the HD recovered as normal with rest. So, they are essentially sacrificing their life force to increase the power of a spell. New spells are gained the same way (but I would hope the choices have something to do with the subclass or background chosen and not just for mechanical power).
Warlock Magic:
This is a kind divine magic, but gained from a powerful being that is not a god exactly, they are actually sacrificing something of themselves, so they are going to expect to be getting something for it. So, spells need to be prepared ahead of time unlike the other 3 classes. Warlock's choose each day from their warlock spell lists, with some spells always being prepared (the ones from the subclasses Expanded Spells). The warlock does not learn spells even though the Warlock description under Pact Magic uses that word, they are granted as boons by the patron. Spells Known becomes Spells Known for that day until you beseech your Patron again for specific powers (you HAVE been good to your Patron, haven't you??). There is no 'upcasting', per se, b/c the spell slots of Warlocks are fixed as it has always been. Being granted new spells (again, they aren't learnt) at higher character levels happens as normal, this unlocks a new and deeper relationship between character and Patron.