Well, I didn't play XI and don't know how dancer was there.

I think the dancer as a stationary, magic character, that would be grounded for most of the time, like this:

High HP and High def/mdef so he could endure a few hits, his dances would be like a regen dance - an aoe regen dance with small potency, an aoe def boost dance, an aoe speed dance and so on... Every dance would work kind of like a sect and it would be active as long as the character is not moving. While this things are happening, the dancer would be able to cast spells as the other healers.

One cool thing is that they could have different animations for the different dances.