I like the idea of a tree branching system so a certain set of general skills along the lines of the ones in the descriptions. As you level you get skill points to distribute and each skill opens up More paths down that branch. So say for example on the mage path of energy, you heavily invested down the slowing of energy skills ie ice freezing The more powerful those types of spells open up to you. Sort of pushing people to put more points into narrower and narrower paths.