Some time back I came up with a hypothetical non-Vancian magic system that could work for a D&D-esque game. I would appreciate feedback either on major problems or minor tweaks.
The key to this is a new resource called "mana". Mana is the reserve of magic that each spellcaster has on hand, and it ranges from tiny to massive. Mana would be gained each level just like HP, with the relevant modifier being Wisdom.
Spell levels still exist in this world, but mean rather different things.
A character learns spells by taking the Learn Spell feat, which allows them to learn a number of spells equal to 1+their Intelligence modifier (minimum 1). This feat can be taken as many times as the character wants. A character can't learn a spell of higher level than their caster level and you can't learn the same spell more than once.
To actually cast a spell they've learned, a character must expend a number of mana equal to the spell's level. Spent mana regenerates at the rate of 1 mana per hour, with a bonus 1 for every 8 hours of rest.
Blindcasting: A character may attempt to cast a spell that they haven't learned by simply concentrating on what they want and letting the magic take its own form. However, doing so doubles the spell's level for terms of cost, which also means that you can't blindcast a spell whose level is more than half your caster level.
Lifecasting: In an emergency, a caster can elect to use his own vis as the fuel for spells. Once per turn, a character may add mana up to his caster level to his mana pool as a free action. However, for each mana added in this manner, the character loses 1d6 life.
Casting from scrolls: If a character has a scroll or other written description of a spell, he may cast it as if he had learned it. This does not cause him to actually learn it, so he will need the scroll again to repeat this in the future.
0-level spells. 0-level spells represent the most basic magic tricks. Anyone with at least one caster level is assumed to know all 0-level spells, and they cost no mana to cast. They would be such things as "light fire", "glowy ball of light", etc
Wizard, warlock, and sorceror would all be combined into a single class which I'm calling "Mage". Mages would get either 2(1d6+Wis) or 1d10+Wis mana per level (I haven't decided which yet) and would have "Learn Spell" as a starter feat.
Bards, paladins, and druids would get 1d6+Wis mana per level. I have considered giving druids and paladins an "attunement bonus" which would probably manifest as certain spells being priced as though they were one level lower, but I haven't decided yet.
Cleric magic should probably stay Vancian, since it actually fits well with the concept of requesting magical favors from a deity.
So that's my magic system. Thoughts?