One long break later
My issue with elements is that they don't fit as well as I would prefer. What among the elements, natural forces, is evil? Again, entropy, natural decay, is really different from necromancy.
I think you need to decide if you want to differentiate casters. Right now, Psi, Arcane and Divine operate nearly the same. (Psi uses points and has attacks and defense. Otherwise, it is pretty close.) The classes have some differences but the mechanics are the same.
So, do you want to have the same thing or different? I think that depends on your world. If arcane and divine operated differently, it would be a completely different flavor. Witch hunts, inquisitions, etc. because how the worked would be different. (Yes, you can have it now but the game mechanics don't help the role playing, since the casters are currently treated the same.)
Okay, the idea of the magic systems being different is confusing. Let me elaborate. I don't mean a new set of game mechanics has to be created for each type of spell. That isn't worth it and not what you want. You can differentiate between the two. Right now, in 3E, all spells use VSM and F or DF for spells. The spells are done the same. What if arcane were VSMF and divine were V and DF? Then, there is a small similarity but a very big difference. Perhaps, taking this further, what if each spell required a drop of blood? The type of blood could then influence the effects of the spell. (From cow to dragon and all in between, including the caster using their own.) This is a big *game mechanic* difference that helps set up many things.
So, to answer your question, that depends. Are they going to be treated the same? Or will they be different?
d20 is all about "balance". (A bad thing, imo.) As you look at the magic system, you will have to figure out its balance and where to put things. With only four elements, casters should probably only know one. With ten, they could know three or four. It all depends on what you want to do.
Okay, this was more abstract than concrete but hopefully it gets you thinking. Again, if it was me, I would do specific specialties of magic. Perhaps, in doing these groupings, you will find certain attributes help. For example, summoning could be ruled by WIS, necromancy by CON and Charms by CHA. Perhaps that is how you will group things.
Just some ideas.
turlough