Fall from heaven 2, a civ4 mod, use magic nodes in an interesting way : the magic you can research is bound to which kind of nodes you control, and the powerful spells need several nodes of the same kind.
The interesting twist is that at the start all the nodes are neutral. And a wizard can build a node amplifer that will give a kind to the node. Once done it can't be reverted. The wizard can only creat a node of a kind that is already controlled. So the only way to learn new way is to build some expensive buildings that gives you new kind of magic.
At the race creation you could also choose magic "affinity", a bit like in MoM.
