there will most likely be skill trees/modifiers in the next expansion. not new skills but ways to modify the ones we already have on a fight by fight basis.
For example take any DoT ability:
1.one option could be the original ability
2.another could be reduced potency of the DoT but receive healing from the ability
3. 3rd option could be remove the DoT and just add its potency onto the initial hit

would mean there is more preperation for battles and rotation may have to be adjusted based on which you use for which fights