Here are a number of different potential systems we could use for spellcasting, with my opinions on each. Thoughts?
1) Casting spells directly affects the spellcaster’s health, inflicting nonlethal damage equal to the level of the spell.
Pros: Flavorful—magic draws directly on the user’s health, or the health of others. Spellcasting ability can be recovered over time as the caster heals, allowing them to cast more spells in a given day, but not all at once. Gets rid of the Vancian (spell slot) system. Uses an existing mechanic and is easy to track. Because mages are crunchier, encourages them to travel with meat-shields/bodyguards and theoretically encourages teamwork.
Cons: Requires redesigning the hp/healing system, to prevent healing magic from instantly restoring spellcasting ability. Makes mages potentially easy to knock out (although this encourages them to surround themselves with bodyguards), since spellcasting damage stacks with normal nonlethal damage. Makes mages extremely reliant on hit points, practically requiring them to focus on Constitution as well as their mental casting stat (Int/Wis/Cha), and makes Toughness a must-have feat. Involves a certain degree of extra bookkeeping beyond the normal spell slot system. Involves redesigning existing feats, class features, magic items, etc. that work with spell slots.
Pro/Con?: Temporary hp make things very interesting—spells such as false life and vampiric touch become much more powerful.
2) Casting spells inflicts nonlethal damage equal to the level of the spell, but this damage is tracked separately from the caster’s normal health and cannot be healed by magic.
Pros: As above, but makes mages slightly less crunchy, and also removes the need to tinker with the healing system.
Cons: Mages are still extremely reliant on hit points and Constitution. Still requires math. Feels somewhat divorced from the “lifeforce” source.
Pro/Con?: Removes the temporary hp “problem”.
3) Spellcasters have a pool of “spell points” entirely separate from their health. Spells draw from this pool. The pool replenishes either 1) once per day or 2) at the same rate as nonlethal damage.
Pros: Allows mages to cast spells without focusing on hp and Constitution—spell points could be based off of casting stat instead. The Psionics system already uses this, so can be converted with some work. Less work tracking hp/nonlethal damage.
Cons: A lot of people really, really hate the spell point system… a lot. Involves a lot of math. Loses the flavor of hp-based magic, and might feel “video-gamey” (even more than normal).
4) Spellcasters have a small pool of “spell points” that is separate from their health, and that recovers almost instantaneously, as in Dragon Age.
Pros: As above, plus less bookkeeping—magic energy recovers immediately. Party can adventure forever, since they’ll have a practically unlimited amount of healing magic available (assuming they have a healer).
Cons: Video-gametastic. Allows casters to “go nova” every single battle, potentially overshadowing other classes. Fun for casters, possibly less for everybody else.
5) We use the normal Vancian spell-slot system as-is.
Pros: I don’t have to do any work. Every existing class, prestige class, feat, magic item, and spell is already designed to support the system. The system can be fluffed to support a number of different concepts.
Cons: Booooring. Less versatile. Lacks the “drawing from life” flavor of Uterian magic.