With the upcoming change to Drain Power and Power Extraction it would be the best time to add a synergy to this skill that would allow our group members to gain increased weapon/spell damage through synergy as it get siphoned away from the enemy. Not only that...
Abilities like Path of Darkness and its morphs are also a really good example of a skill that could use some well rounded synergy that would drastically improve its usefulness.
Aspect of Terror being much weaker than the new Turn Evil, could also gain an interesting synergy.
Other theoretical candidates, since we would still be missing something in the assassination tree would be Mark. Instead of a long debuff, it could become a short debuff, a powerful dot/utility-like skill boosting damage, and with such a change gain another interesting synergy.
About every single class has some form of reactive synergy in this game, apart from NBs. We only get ours from our 2 ultimates, which rarely see any real use due to there being much more powerful or desired ultimates.
What do you think, folks?