Magic development systems.

In Eador, there’s a turn-based strategy game where the magic system was implemented in a rather interesting way. You can attach a certain number of different school extensions to the guild itself, and each extension, as you might guess, provides spells from that school. Moreover, if only a couple of magic schools are available at the first level of the guild, the maximum number of available extensions decreases at higher levels, down to one at the maximum guild level. In my opinion, this is a pretty good system.
But I voted for the system from the fourth game.