Spontaneous casting 'cost'

[Kisanji Arael]

For a first level spell? Sure.

Is it balanced? According to a statistically correct D&D? No.

But!!!

Do you trust your player? Yes, you obviously do. I would just say that as long as it's a non-damaging spell (or armor providing) then it is probably fine. Hell, it's the only way that hold portal would ever be useful anyway. But make sure that he clears any spells he plans on using in this way on a case by case basis beforehand.

 

[airwalkrr]

This ability should definitely require a feat. I suggest the following feat.

Instant Cognition
Prerequisites: Spell Mastery, ability to prepare spells
Benefit: You may leave one or more spell slots open for the the ability to instantly memorize a spell later. This effectively allows you to spontaneously cast a small number of spells using the open spell slot. You may only cast spells that you know with Spell Mastery this way, and the open spell slot must be at least one level higher than the spell that is spontaneously cast. For example, Mialee has left a 4th-level spell slot open. Among the spells she knows by virtue of Spell Mastery are magic missile, grease, scorching ray, stinking cloud, and ice storm. She could use that open spell slot at any time to cast magic missile, grease, scorching ray, or stinking cloud, just as if she had prepared it, but not ice storm. Use of this ability is a swift action.
Normal: It takes at least 15 minutes to memorize a new spell to fill an empty spell slot.

 

[RisnDevil]

ummmmm....

It was mentioned earlier, though the post (by Nifft) seem to have been ignored, so let's try again. In The Player's Guide to Faerun (and POSSIBLY FRCS) there is a feat called Signature Spell (which is a requirement for Innate Spell, I think). Signature Spell requires Spell Mastery of the chosen spell, and allows you to convert other spells, of equal or higher level, into that chosen spell. (Sorry if I have said more than OGL/OGC allows, since I do not believe this feat is either.) This sounds exactly like what you want.