I think a tangential question is in order: How many classes can they fit in the book?
If it's a standard, PHB-sized one (3.X/4E style), I think we could see anywhere between 9 and 12 classes depending on the space allowed for each.
I don't think more classes are going to take up too much space. Let's look at 3.5's page count, by chapter.
Abilities: 4 pages
Race: 10 pages (7 races; we could stand to have a few more)
Class: 40 pages (9 classes, multiclassing, experience and levels; we could stand to have a few more)
Skills: 26 pages (we'll definitely have fewer)
Feats: 15 pages (we might have more, depending on how it works out)
Equipment: 22 pages (probably about the same)
Combat: 27 pages (depends if the tactical combat rules will be in this hypothetical book)
Adventuring: 9 pages
Magic: 13 pages (just to explain how spellcasting works. Uh oh...)
Spells: 123 pages! (15 pages of spell lists, 108 pages of spells, too many spells to count)
Wow, I think we found our problem. How did the other editions do? (note: I only have access to retroclones, so the actual printings are probably different; the principle is still important though)
OSRIC: 82 pages of spells (4 classes)
Dark Dungeons: 42 pages of spells (2-ish classes)
Labyrinth Lord: 24 pages of spells (2-ish classes)
Let's look at an example spell, to see the difference in presentation. Let's try
bless. [sblock]Here it is in Labyrinth Lord:
Bless (reversible)
Level: 2
Duration: 6 turns
Range: 60'
Bless fills the caster's allies with courage, but does not affect enemies within the affected area of 20' x 20'. Each ally gains a +1 morale bonus and +1 on attack and damage rolls.
The reverse of this spell does not affect allies within the effect area, and incurs penalties of –1 instead of bonuses to the rolls indicated above.
Here it is in Dark Dungeons:
Bless (R)
Cleric 2, Druid 2, Shaman 2
Target: All allies in a 20’ radius
Range: 60’
Duration: 1 hour
When this spell is cast, all friendly creatures in the area of effect gain a +1 bonus to attack, damage, and morale rolls for the duration, providing they are not already engaged in combat.
Reverse: Blight gives each enemy in the area a –1 penalty to attack, damage and morale rolls for the duration providing they are not already in combat. Each of these enemies may make a saving throw vs Spells to avoid the effect.
Here it is in OSRIC:
Bless (Reversible)
Clerical Conjuration/Summoning
level: Cleric 1
Range: 60 ft
duration: 6 rounds
area of effect: 50 x 50 ft
components: V,S,M
casting time: 1 round
Saving throw: None
This minor benison raises the morale and attack rolls of any creatures allied with the casting cleric by +1 or +5% as appropriate. Any creatures already engaged in melee combat will not gain the spell’s benefits. The spell is reversible, allowing the cleric to curse his or her enemies with –5% morale and –1 to hit.
The spell’s area of effect is a circular area centred at the target point, which cannot be more than 60 ft from the cleric. Holy water is required to cast the spell (unholy water for the reverse).
Aaand here it is in 3.5:
Bless
Enchantment (Compulsion) [Mind-Affecting]
Level: Clr 1, Pal 1
Components: V, S, DF
Casting Time: 1 standard action
Range: 50 ft.
Area: The caster and all allies within a 50-ft. burst, centered on the caster
Duration: 1 min./level
Saving Throw: None
Spell Resistance: Yes (harmless)
Bless fills your allies with courage. Each ally gains a +1 morale bonus on attack rolls and on saving throws against fear effects.
Bless counters and dispels bane.
Bane
Enchantment (Compulsion) [Fear, Mind-Affecting]
Level: Clr 1
Components: V, S, DF
Casting Time: 1 standard action
Range: 50 ft.
Area: All enemies within 50 ft.
Duration: 1 min./level
Saving Throw: Will negates
Spell Resistance: Yes
Bane fills your enemies with fear and doubt. Each affected creature takes a –1 penalty on attack rolls and a –1 penalty on saving throws against fear effects.
Bane counters and dispels bless.
[/sblock]Noticing a pattern here. Let's take a look at another spell. Let's try
binding. Here's how it looks in 3.5 (narrow your browser window for full effect):[sblock]Binding
Enchantment (Compulsion) [Mind-Affecting]
Level: Sor/Wiz 8
Components: V, S, M
Casting Time: One minute
Range: Close (25 ft. + 5 ft./2 levels)
Target: One living creature
Duration: See text (D)
Saving Throw: Will negates; see text
Spell Resistance: Yes
A binding spell creates a magical restraint to hold a creature. The target gets an initial saving throw only if its Hit Dice equal at
least one-half your caster level.
You may have as many as six assistants help you with the spell. For each assistant who casts suggestion, your caster level for this casting of binding increases by 1. For each assistant who casts dominate animal, dominate person, or dominate monster, your caster level for this casting of binding increases by a number equal to one-third of that assistant’s level, provided that the spell’s target is appropriate for a binding spell. Since the assistants’ spells are cast simply to improve your caster level for the purpose of the binding spell, saving throws and spell resistance against the assistants’ spells are irrelevant. Your caster level determines whether the target gets an initial Will saving throw and how long the binding lasts. All binding spells are dismissible.
Regardless of the version of binding you cast, you can specify triggering conditions that end the spell and release the creature whenever they occur. These triggers can be as simple or elaborate as you desire, but
the DM must agree that the condition is reasonable and has a likelihood of coming to pass. The conditions can be based on a creature’s name, identity, or alignment but otherwise must be based on observable
actions or qualities. Intangibles such as level, class, Hit Dice, or hit points don’t qualify. For example, a bound creature can be released when a lawful good creature approaches, but not when a paladin approaches. Once the spell is cast, its triggering conditions cannot be changed. Setting a release condition increases the save DC (assuming a saving throw is allowed) by 2.
If you are casting any of the first three versions of binding (those with limited durations), you may cast additional binding spells to prolong the effect, since the durations overlap. If you do so, the target gets a saving throw at the end of the first spell’s duration, even if your caster level
was high enough to disallow an initial saving throw. If the creature succeeds on this save, all the binding spells it has received are broken.
The binding spell has six versions. Choose one of the following versions when you cast the spell.
Chaining: The subject is confined by restraints that generate an antipathy spell affecting all creatures who approach the subject, except you. The duration is one year per caster level. The subject of this form of binding is confined to the spot it occupied when it received the spell.
Slumber: This version causes the subject to become comatose for as long as one year per caster level. The subject does not need to eat or drink while slumbering, nor does it age. This form of binding is more difficult
to cast than chaining, making it slightly easier to resist. Reduce the spell’s save DC by 1.
Bound Slumber: This combination of chaining and slumber lasts for as long as one month per caster level. Reduce the save DC by 2.
Hedged Prison: The subject is transported to or otherwise brought within a confined area (such as a labyrinth) from which it cannot wander by any means. The effect is permanent. Reduce the save DC by 3.
Metamorphosis: The subject assumes gaseous form, except for its head or face. It is held harmless in a jar or other container, which may be transparent if you so choose. The creature remains aware of its
surroundings and can speak, but it cannot leave the container, attack, or use any of its powers or abilities. The binding is permanent. The subject does not need to breathe, eat, or drink while metamorphosed, nor does it age. Reduce the save DC by 4.
Minimus Containment: The subject is shrunk to a height of 1 inch or even less and held within some gem, jar, or similar object. The binding is permanent. The subject does not need to breathe, eat, or
drink while contained, nor does it age. Reduce the save DC by 4.
You can’t dispel a binding spell with dispel magic or a similar effect, though an antimagic field or Mordenkainen’s disjunction affects it normally. A bound extraplanar creature cannot be sent back to its home plane due to dismissal, banishment, or a similar effect.
Components: The components for a binding spell vary according to the version of the spell, but they always include a continuous chanting utterance read from the scroll or spellbook page containing the spell, somatic gestures, and materials appropriate to the form of binding used.
These components can include such items as miniature chains of special metals (silver for lycanthropes, cold iron for demons, and so forth), soporific herbs of the rarest sort (for slumber bindings), a bell jar of the finest crystal, and the like.
In addition to the specially made props suited to the specific type of binding (cost 500 gp), the spell requires opals worth at least 500 gp for each HD of the target and a vellum depiction or carved statuette of the subject to be captured.[/sblock]And here's how it looks in previous editions.... Wait, what? It isn't even in previous editions? So, did the authors of 3rd Edition just decide for no reason to make up an insanely long spell description and stick it in the spell list?
Yeah, I think we found the problem.