Personally, I think that since wizards study their spellbooks every single morning, they should be able to cast in a manner similar to sorcerers, but that's just me. The whole 'spells per day' schtick just seems.....well.....fake. I'd like to implement a sort of 'mana' system that I've been working on and tried a version of in a past game. It's mana-based, but keeps the divisions of power and style between the sorcerer and wizard. It goes sorta like this:
For each level of each spell you would have per day, you get one mana point. So if you would normally have (bonus spells from high Int included) as a wizard (not mechanically exact for SPD, just for example purposes, ) 4 0-lvl spells, 3 1-lvl spells, 2 2-lvl spells, and 1 3-lvl spell, you would have (1x3=3, 2x2=4, 3x1=3; 3+4+3=10) 10 mana points. Casting a lvl 1 spell costs 1 mana point, a lvl 2 spell costs 2 mana points, and a lvl 3 spell costs 3 mana points, etc. Cantrips are free, you can cast as many as you want. So the casters have something to fall back on at the end of the day(however weak it may be) and can do their various "hey, I'm a wizard, I can make stuff float!" stuff.
With this variant magic system, wizards can be like the classic wizard "I know this spell so I can cast it, yay!" instead of the D&D "I know this spell but I can't cast it because I chose Mordenkaiden's Magical Washbin today! Drat!". The wizard gets the flexibility of the sorcerer, but still has to study his spellbook every morning. But the sorcerer still has the advantage, since while he can't cast such a wide variety of spells, he cast cast them a lot more, and doesn't have to study a book.
Clerics follow a similar system, but with spirit rather than mana.
As for epic spells, I don't rightly know what I'll be doing for that. Probably keep it the way it is, a certain # of Epic spells per day, but you can cast any epic spell you know, provided you fill any requirements set forth in the spell.
I find that this speeds up the game and makes it a lot more simple for spellcasters, although I see two significant holes that need to be filled up.
Clerics and their Spontaneous Casting - how to make up for this...perhaps rule that since clerics are so much higher powered than the other classes in the first place, the so-called 'hole' is irellevant and should be ignored.
Sorcerers and their Spontaneous Casting-Style Magic - While I think this system as a whole works out nicely, I'd like to add something to let the sorcerer keep that little something that sets him apart from the wizard. Wizards still need their spellbooks while sorcerers don't, but they still need that little bit of "Hey we have a significant advatage over you NYAA NYAAA!"
Any suggestions for an sorcerer's advantage, people?