Background

In 5E spellcasters either know spells or prepare spells.  These describe a selection of spells on the caster’s spell list which they can actually select for casting at a given time. The primary difference between these categories is how often this list of castable spells can be changed: a caster can change their prepared spells after completing a long rest, but can only change known spells when they gain a level (and in fact they can only replace one known spell per level gained). In either case, a character can only have a specific number of spells known or prepared at a given time based on their class.

There’s an exception to this: wizards (of course). Wizards prepare spells, but they do so from their spell books. Spell books act as bit like a list of spells known except there’s no limit on how many spells a wizard can have in their spellbook. In addition, while a wizard adds 2 spells to their spellbook every time they gain a level, they can also add spells they find in play.

So our models are:

1. Spell list -> spells known -> spells cast
2. Spell list -> spells prepare -> spells cast
3. Spell list -> spells in spellbook -> spells prepared -> spells cast

Analysis

Models 1 and 2 each have their own problems. In model 1, spells known being difficult to change as well as very limited pushes players toward monotony, a pattern of selecting only those spells they think are optimal; those who are plugged into online communities of optimizers will be more likely to align with community lists of optimal spells. In Model 2, spells prepared being able to draw upon a class’s entire spell list can easily cause analysis paralysis or lead players to monotonous selections to avoid that issue; they’re also pone to power inflation issues as any publication which increases their class’s spell list immediately gives them more power that other characters don’t get.

I think we want to use a hybrid approach like the wizard’s for all spellcasters. This allows us to expand a class’s spell list without worrying about all those new spells instantly showing up for characters to use. Players will then have to filter the list of spells their character can use at all down to an even narrower list of those they’re ready to use in a given session. In effect, we’d make every caster have both spells they “know” (for wizards, those they have in a spellbook) and spells they have prepared.

Research

Spells known/prepared by class (approximate):

• Bard: Level + 2
• Cleric: Level + Wisdom modifier
• Druid: Level + Wisdom modifier
• Paladin: 1/2 Level + Charisma modifier
• Ranger: 1/2 Level (rounded up) + 1
• Sorcerer: Level +1, eventually slowing way down after 12th level
• Warlock: Level  + 1 through 9th level; + 1/2 Warlock level beyond 9th level
• Wizard:
  • Prepared: Level + Intelligence modifier
  • “In spellbook” (i.e. known): at least 6 + 2/level. Also any spells found in play

The number of spells prepared seems to be fairly standardized at spellcaster level + modifier for spellcasting attribute; Paladin uses 1/2 level because they’re a half-caster (that is, their access to new higher levels of spells happens at 1/2 the rate of full casters like Clerics, Wizards, and Bards. We can take that as our general formula, though ideally we’d express it in a way that doesn’t require learning weird exceptions for some classes. 

A wizard starts off knowing about 50% more spells than they can have prepared, and automatically gains spells known at twice the rate the number of spells they can prepare increases. Taking the wizard as a guideline, we should have characters know about twice as many spells as they can have prepared. The wizard’s ability to exceed this limit with spells found in play and transcribed intro their spellbook can be treated as a distinguishing feature of the class.

Proposed solution

Let’s add the following definitions:

• Casters cast spells from those they have prepared 
• A caster can prepare a number of spells for a class equal to their caster level in that class + the class’s casting attribute modifier. They prepare those spells from the list of spells they know for that class. They can change the spells they have prepared after a long rest. 
• A caster can know a number of spells for a class equal to twice the number of spells they can prepare for that class. They can only know spells from the class’s spell list, and only those of a level up to 1/2 their caster level in the class, rounded up.
  • A character’s known spells for a class only change when the character gains caster levels in that class – a Paladin gaining Warlock levels can’t make any changes to the Paladin spells known. When a character gains a caster level, they can:
    • Learn 2 spells from the class’s spell list of a level they can know for that class
    • Change 1 spell already known from the class’s spell list to another spell known from the same list of a level they can know for that class.
  • In place of known spells, Wizards have spells recorded in their spellbooks. These are the same as known spells, except:
    • A wizard can’t replace a spell in their spellbooks when they gain a level.
    • A wizard has no maximum number of spells recorded in their spellbooks (though any one spellbook of the wizard’s might not be able to contain all their spells; they may need to have multiple spellbooks).
    • A wizard can add spells to their spellbooks without gaining additional wizard levels; assuming they have a source for a wizard spell not already in one of their spellbooks (e.g. a scroll of wizard spells, another wizard’s spellbook another wizard willing to teach them the spell) they can add the spell to their spellbook in play or during downtime.
• A character’s caster level is defined for a specific class. Their caster level in a class equals:
  • Class level for Bard, Cleric, Druid, Sorcerer, Warlock, Wizard, or any other class which can cast up to 9th level spells
  • 1/2 class level (rounded down) for Paladin, Ranger or any other class which can cast up to 5th level spells but not 6th or higher level spells
  • 1/3 class level (rounded down) for Eldritch Knight, Spellthief, or any other class which can cast up to 4th level spells but not 5th level or higher level spells