the thing is actualy that there isn't much left to add at this point.

you could always add longer and/or more complex combos, but that would make the game overall harder for quite a few ppl I guess. same happens with more stances and buffs you need to keep up.
for tanks you could probably add more damage potential but i you give one of them a new defensive cd, the other will need a similar skill aswell. and more def cds would just trivialize tank busters.

though new skills will very likely mean a change in your rotation so some older skills are probably not be used anymore.

new traits is probably the better option but that means less fun compared to new skills since you are basically playing the same game you did before