What do people think about the idea of spell prerequisites? Like how feats have trees, so should spells--lower-level spells lead to higher-level versions of the same theme. I think that would help balance some, and I like the flavour of it better. As it is now, I can spend eight levels practicing throwing fireballs, and then at my next level I can suddenly raise a sizable number of undead, with no previous practice raising even the weakest undead.
This would go better with non-scaling spells, I think.