Well, just rewriting the spells where needed, bumping spells up or down levels, and so forth would basically fix casters.
Fly and blast, confound and transform, is fun, so that should be retained. Magically picking locks or detecting traps... where did that come from? How does that even make any sense? What sort of forces are being manipulated there?
Fabricate and Creation being so much more powerful than actual skills is kind of a problem, too... basically, things I'd exclude from spells are:
- anything stolen from psionic archetypes. No mind reading, no telekinesis and so forth.
- anything that gives a crunch benefit without any sort of a fluff reason. How would someone even research a spell like that?
What I'd add would be:
- Spells still requiring skill checks. If you move twice as fast, you must have some basic competence in balance, to remain standing, for an example.
- Shapechange should at least require a decent skill check to know about the monster. So no turning into types you don't know about.
Besides that, shifting spell levels would do quite a bit. Any spell that's a must-have or obviously best for it's level is too strong, although a spell that retains its usefulness over the career of a character is just fine. Basically, it's not that Meteor Swarm is underpowered, it's just that gate, time stop and shapechange are overpowered.
You know, Paizo did have one neat idea in their brand of polymorph nerf, although Wizards also has similar spells -- spell 'trees' where a higher-level spell encompasses multiple lower level spells.
Especially if you add the prerequisite of knowing those lower-level spells, or something similar....