Let me start by saying, before the flame, that it seems like square has shifted away from the support class. Thus the jack of all trades could do anything. I also think we need more tanks and more variety of them.

RDM lacks interesting/unique skills for Healing or DPS. Mostly because of overlap from other disciplines. I do see an opportunity for some of their classic moves to help them be a tank.

RDM would surely be the best magic defense tank. They would also have good range tanking abilities as well. They would be better at AOE tanking in general and have a harder time tanking single, hard hitting, targets.

RDM would rotation would revolve around magic spike spells and "En" Weapon enhancing spells. Instant cast and debuffing would also be woven in.