i was wondering if you can give some thoughts/rules for magic. limits or maybe source of magic. Does it come from external source or internal. or both? just trying to figure out a system or different branches of magic that might have different sources or use magic differently.  

eg. necromancy might drain a users vitality due to giving life to animate the dead so they need to steal vitality from other sources and along that road they need more sacrifices or they refine the vitality or learn to store it up for later use.

then summoning magic might involve binding creatures to your soul. so if the creature is slain then it goes into a dormant state but doesn't actually die because it is anchored to the summoner's soul. The summoner would have to harmonize his soul with a creature so there could be issues with compatibility so there could be something creatures that one could not bind with.

Then enchanting would be different. i was thinking of using magic runes of different tiers and this is more like word magic where you can enhance objects and people but using a combination of symbols. this magic would be determined by one's knowledge but the issue becomes how does one learn new runes? or perhaps they know all the runes and have to experiment by combining them for different effects.

those are just some ideas. nature magic would be different as well as evocation (energy based magic) 

should we just let people decide their own magic and it's rules? would be easier and faster i guess

(Edit): Unless magic always existed but the practitioners remained hidden until the barriers started crashing down. then it was up to the magic users to teach the normal people in ways to defend themselves. I figure it is safe to assume that creatures of heaven and hell should have a large understanding of magic already. As well as having magical devices and relics.