^^ I hope 0-levels are still in in some form ... cantrips are fun!
My prediction: spread current and epic spells over 30 levels; one charcter level = one spell level. So a fifth level wizard can cast level five spells and below. Current spell lists are split, so that, for example, less powerful 1st level spells are still 1st level spells, and more powerful 1st level spells are 2d level spells. Current 2d level spells become 3rd & 4th level spells, 3rd become 5th and 6th, etc.
Now here's the trick ... at level N, a certain number A of (N-x) level spells are at will; a number B of (N-y) spells are per encounter, and a number C of (N-z) spells are per day -- where x > y > z and A > B > C. I'd bet 0-level spells rapidly become "at will" abilities. The question is how many of each A/B/C, and what ratio of x/y/z to make the system balanced, keep things interesting, but not make wizards too overpowered. For example, you might start with A = Level + ability mod, B = 1/2 Level + ability mod, and C = 2 + ability mod with X = 4, y = 2, z=0 (and let's assume zero levels go to 5 + ability mod). With those number, a 5th level wizard with Int 16 (+3) would likely have:
- 8 "0-level" spells at will
- 8 "1-level" spells at will
- 5 "2- and 3-level" spells per encounter, and
- 5 "4- and 5-level" spells per day
As you rise in elvel, the number of "at will" abilities increases slightly, but you can increase them in power; the net number of per encounter and per day abilities increases less quickly but they can also increase in power.