I personally like when people have to make choices for their character that can differentiate their builds and skills from others. For example the choice between a vamp a werewolf or neither. I think a cool addition the to mage's guild and fighter's guild skill line would be how you level up and perhaps just add a few more passives or even activated abilities for people to level up to. Anyway the skill lines would level up depending on how you play, the more abilities you use that cost mana, the higher your mage's guild skill gets, and vice-versa, the more stamina based abilities you cast the higher your fighters guild skill level gets. And makeit take a while to level up so you have to make a conscious decision as to which skill line you want to level up. Just a cool idea I think.