I’m also looking at developing my system. Being a big fan of Rolemaster, I am looking to have a large number of specialists, the ability to have some crossover and a very wide variety of spells both in raw number and function. One way in which Rolemaster cheated in terms of number of spells was to make a few basic spells and then have more powerful versions at higher level. You would have Bolt I at level 5, Bolt II at 8, Bolt III at 20 or something to that effect. This works best if you have players who do not wish to tinker with spells. If the players want more of a hand at it, let them develop their own high level spells, just provide them the beginning framework.
One thing that I would avoid, is to have every branch of magic cover every need. Make one or two branches that have a little bit of everything but have others that focus on one thing and do it the best. A Seer could make a great character, sure they don’t blow things up or shield the party but they have fun utility spells that drive the GM nuts. A Necromancer can do some things like a Seer they just can’t do it as well but, they sure can summon. However, for raw damage they can’t touch an Elementalist.
Without focused specializations you have a bunch of magic schools that simply mirror each other. The players and NPC’s will be casting spells X, Y, and Z all from different schools of magic and realize that they are really the same spell. Spice it up by emphasizing strengths and weaknesses.