Work In Progress
Interesting topic.
I am currently working on honing a home brew campeign's magic system from the core DnD rules, and am having some fun with it.
I find when I am worldbuilding I like to get one idea and then flesh it out to cover all metaphysical world elements.
On to the current example:
I have removed the traditional DnD Abjuration, conjuration, etc, and have recategorized the mage spell list into ten groups based upon the colour spectrum. Each colour represents an aspect of the universe, and some colours are "opposed" in the manner of a standard duality.
This idea has flowed on to having me make the seven base colours of the spectrum plus brown, white and black becoming the building blocks of the universe. The origins of gods are determined from the origins and nature of each of the colours.
It also means I can do the "Final Fantasy" Red mage et al with magical specialists...
In terms of how this affects divine magic, I have decided that divine magic is similar to arcane magic but is more a magic of allegience as opposed to a magic of usage. So, for example, a priest of a god alligned to the Yellow (symbolized by the concept of Aura) would have the ability to attune auras to heal, whereas a mage of the Yellow would have the ability to manipulate auras to create illusions, or at the higher level, to alter or create material things.
I have never been overly comfortable with psionics in DnD, I think it makes DnD too much like paladium. Not that there is anything wrong with that...
I have basically rescripted the concept of Psionics to being the same as that of a cleric without a god... so if players want it, they can have it that way. I am in the process of creating appropriate Spheres (so instead of War, Travel or Luck, they might choose Coercion, Psychokinesis or ESP). None of my players are Psi heads so this is kind of a non issue at the moment.
One interesting side effect of this particular dose of worldbuilding is that I have decided that this world does not really hold scope for the standard alignment system, and instead a character has a blend of the colours within them that fits the allignment concept. Some spells (especially the allignment based ones) need some alteration to fit this idea, but it actually works remarkably well.
I guess the point I am making is that a concept as fundamental as "what magic system do you use" is IMHO a really big question with a heck of a lot of implications, and really needs to be thought through. I have been running a game in this universe for some time, and this is still very much a work in progress.
MoFo