They can totally add a combo system like in PvP and add a new panels of skillsets in PvE.
If it works like in Aion (the use of 3 buttons for alternatives skills).
In my opinion, it would be really cool to add a system like this in PvE, it would make the game less nervous, but more strategic.
Mages could have also channeling skills, where the most you stay input the more the damage/healing/etc when you release...
You could have for example; "provoke" followed by 3 choices: "Q" = stun bash | "E" = Flash Heal | "R" = AoE Provoke/flash , etc...
The actual combo system is just a "mandatory" loop... it would be great to change the way to play that could make us exit from that loop...