When all spells are spent, when all mana is gone, wizards are a bit in a meh situation. While it’s cool that you want to let the magic go out because of careful strategy, I think giving risky opportunities to players is also good.
So the idea is that casters and wizards can overcast. I’ve been using these rules for Mageblade! but you can adapt them easily. I swear the Game Police won’t come after you. I suggest you try this with a lot of limitations first, and then try to relax the limitations a bit and see what works. If the player keeps on overcasting too much for your tastes, just require extra rolls, or add limitations.
This is what works for us: you need to find what works for you.
- The caster, first, must choose the spell to be overcast. You can limit the spell to only spells the caster has not cast today yet, or to spells the caster had memorized, or only for granted powers, or only for patron spells, or for devotions, or domain spells. Try to find out what you feel is right for your game.
- The caster must then roll to cast the spell right:
- for knowledge-based casters, like MUs, wizards and MB Caster, find if they can control the spell rolling under intelligence. If the roll is failed, the caster botches: lets out a magic blast (3d6 damage in 10′, save for half), or a catastrophe from W&W, or use one of those GURPS magic critical failures tables.
- for introspection-based casters, like mageblades, clerics, and even sorcerers (do not forget that sorcerers can’t read, or else they would be wizards), find if they can find clarity rolling a wisdom check. If the roll is failed, the spell simply does nothing, but the spell can’t be overcast for a while. Maybe until penance is done. Maybe until sleep. Maybe until drugs are taken. Or maybe one hour of meditation is enough.
- The caster must then roll to find the power to cast:
- for casters that are fueled by their belief in gods or power-granting patrons (like the clerics, mageblades, paladins and casters devout to a god, DCC casters with patrons), roll under charisma. On a failure, the Power that Be is displeased with the lack of self-reliance of the requester, so not only does nothing, but you won’t be able to cast for a while. Cue sacrifices, fasting, pilgrimage, etc.
- for casters that provide their own mana, like MUs, wizards, and MB casters or Mageblades that are not godbotherers, find if they manage to gather the power without consequences rolling under charisma. On a failure, the power is still harnessed but the caster is zonked out, collapsed for 1d6 turns. Healing spells do not really fix this much.
Option: Recovery with Surgery & Chemistry
While a zonked out character will be out cold for a while, you can have a surgeon do an emergency procedure involving a healing potion and some horrible surgery you you not want to know. It takes one round, one healing potion and one surgery/healing roll. If the roll is successful, the caster recovers the round after the surgery.