Alternately, do the 4E synchronization treatment to earlier versions, but do it the classes instead of spells. After all, the confusion of a 5th level wizard getting 3rd level spells can be solved either way.
So treat cantrips as spell level 1 instead of zero, and slide everything else up a notch (i.e. fireball is now a 4th level spell). Or if you can't stand this, keep cantrips as freebies, level the spell levels largely alone, but have 10 of them. Move individual spells around to flesh out the 10 levels, sliding more powerful ones up a notch. Wish become 10th level.
Then make all classes 10th level. A 4th level wizard can cast 4th level spells.
Obviously, you have to cram a lot more into each level, but could probably work around this with some creative multiclassing for breadth (and solve a lot of multiclassing issues at the same time). As long as it takes the kind of effort that people expect for that wizard to get to where he can cast fireballs, it's all just numbers.