Oooor, we could do it ffx or ffxiii style where you kinda allocate points to stuff on a grid, eventually reaching up to a new spell. Considering its an mmo we could make the points needed to get certain spells or go on certain skill paths really tough so people dont get everything and "customizing" your skill path becomes a thing. Also considering its an mmo and most pple would eventually end up going the "preferred" skill path, we could make the different ways to go have their own exclusive merits, making it very situational and debatable which to choose.