Skills With Prerequisites
Right now all skills are gotten in a lineal way you only need certain level to get them. What if you make that into an skill tree like Diablo or WoW ones, where you can only pick an skill when you have a certain combination of others: ie, to geet rook's boulder you should get divine strengh 1 plus lvl 2 hammer.
Once the game has a wider skill variety for every hero, taht would be a good way to impede metagame-breaker builds.