I see quite a few number being thrown out here (which I'm not going to look in to because I don't have the data at my fingertips), but would it really be necessary for an evasion tank to have much more HP than our current mitigation tank (PLD), even in cloth armor? If evasion chances were high enough, then normal hits wouldn't one shot them, and there would be plenty of time to heal between hits (this is on average, there are CDs for those times when the RNG hates the RNG tank). Then for heavy hits, they would have to have abilities just for those. I was thinking along the lines of what PiedPiper had mentioned
prevents any attack which hits the ACDC for say, 30% or more of its total health from killing it (leaving them with 1 hp, next attack would kill of course since the ACD now has less than 30% hp to loose).
for hits like HM Titan's table flip, which would reduce them to 1 HP, and then HOPE that you can dodge any hits before you get healed, which would have to come fast.

My concept of the abilities for an evasion tank, mostly consist of additional evasion, and the ability to flat out survive attacks, like survive with 1HP after heavy damage.
1. a static ability to increase enmity and evasion, like sword/shield-oath
2. a long cool down ability to survive with 2-10 hp after any hit would reduce you to that little
3. a reasonably short cool down or two to survive with 1HP after taking a large percentage of their, or some other condition.
4. an ability combo that increases evasion temporarily
5. and maybe another cool down, also, for increased evasion.

Of course, any class has more than 5 skills, but, something like this is what I imagine.

Of course, there are some things to be careful of: undodgeable attacks, party-wipe skills (although if everyone else dies, it probably won't make much difference), and the ability to reach 100% evasion for any extended period of time.

A note however, I would put dancer and ninja in DPS roles, with dancer having some support skills and ninja having some debuff skills. I like the idea of a evasion/ranged/magic tank, just seems different (kinda like a melee healer).