So, if clerics can benefit from some spontaneous choices, couldn't wizards do the same? Maybe. I'm willing to give it a try.
Seems to me that if wizards didn't feel like they always had to have Fireball or Sleep memorized, they might just memorize Gust of Wind or Mount.
My thought was that (whether you use Schools of Magic or not) wizards could attach themselves to a school and in doing so, they would get access to certain spells to use "in exchange" for any spell they have memorized.
Of course, you can always exchange a higher level spell for a lower level spell.
I've tried to match a "theme" to the wizard school. I think these groupings have a lot of potential. I also think that Invocation/Evocation will be a popular choice. :)
|Protection from Evil
|Protection from Cantrips
|Monster Summoning 1
|Wall of Stone
|Contact Other Plane
As always, YMMV. I have not playtested this yet, but I'm planning on it.
EDIT: I'm going to make wizards roll INT or less on d20 to change a spell on the fly. If they want to change it in memory, they can do that over the course of several turns -- no problem. Clerics (because of "divine favor" make their swaps at will.