balancing the classes is imho a bad idea from the start. in ADOM they are not just "unbalanced" by chance. they are supposed to be that way. (*cough* acid ball *cough*)
the idea about mutually exclusive spell is to simple and have no reasoning behind it. i could see it as some sort of specialization in various schools but this idea needs lots of thinking - just throwing it "as it is" is not enough :P
the skill system in diablo II was not so good imho lvl prerequisites for spells are just unnatural.
maybe ... hmmm... skills for various schools of magic could serve for stating prerequisites. thats more natural when using some spells makes u increase ur skills with habdling them and thanks to that u can master more complex spells im to lazy to dig through the news and website to search if creator already did something with magic ;>