Let´s not call them spells at all. We could call the Abilities (innovative, eh ?). Group them into a couple off different ranked lists, where you have to take a certain amount of low level ranks before you can access the higher ones.
That's a good start. A thing I'm working on with Paul S. involves going completely skill based, and then having abilities of all types be learnable kind of like D&D wizard spells if you meet the skill prereqs and either gain access to them in play (such as by finding a teacher or a spellbook), or by taking extra time to research them yourself.
The major dividing line, therefore, is between permanent changes to yourself and the acquisition of new available actions. Permanent changes don't cause drain and are on all the time - and take longer to learn and have higher skill requirements than actions.
So you can learn the spell "Fire Aura", or you can just "learn" having a Fire Aura all the time. The first drains you and has a limited duration when used, the second is more annoying when wandering through caligraphy shops, has higher prereqs, and takes longer to master.
It's a magical world, I don't see why gaining a couple extra arms isn't something that you could just learn.
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From a game balance standpoint, game balance is never ever going to work if some people only ever gain new abilties when they go up in level and some people gain new abilties just by visiting a library for a few days. The only solution is either to have noone gain abilities except through level gain, or to switch everything over to the model presented by Wizardous Spell Acquisition.
So then we note that only one of those allows us to model people being able to be taught how to prepare the nardoo fern without death by beri beri, and then we make Whirlwind Attack by gainable in the same way as Fireball is - assuming you meet the prereqs for it.
Taking your point about costs and drain form another thread, what kind of of drain system would be most appropriate for D20 ?
We are switching damage over to static damage levels with soak rolls, and drain is going to cause
fatigue - a separate tally of damage levels which cause penalties to accumulate and eventually make you pass out (but don't kill you under any circumstances).
With straight D&D you'd probably want to make it special stacking penalty that applied to most things you were going to do -
including your spell penetration and save DCs.
And most importantly, would there be away to quicky implement this without throwing most of the existing material overboard ?
You could do it something like this:
After casting any spell, make a Will Save with a DC of 13 + 2x Spell level (or some such number). For every two points you fail the Will Save by, you take a -1 penalty to all attack rolls, skill rolls, spell penetration checks, and save DCs for 1 minute per level of the spell (minimum five rounds).
Or you could just Daze people for one round if they use a spell which wasn't available 4 levels ago - but that involves believing that a 7th level spell is powerful and a 5th level spell isn't - and that's absurd.
Then you'd need to dump all the spells that give long duration benefits and rework them so that they have the same cost/benefit ratio as class based benefits - but you have to do that anyway. Then you'd have to switch Druids, Clerics, and Bards over to a Wizard-like spell acquisition system (you have to do that anyway), and then the spontanaity of the new Wizard would leave the Sorcerer completely redundant and you could drop it altogether.
-Username17