I don't really see how Ninja could end up tanking if Utsusemi was like XI. If they have Gladiator or Marauder for cross-class skills perhaps, but other than that tanking with a DPS (that isn't Summoner) would just be too annoying to bother pulling off. Everyone would have to watch their enmity to keep the Ninja at top. I find it much more likely that Ninja will take from Pugilist, Lancer, or Archer though. I seem to remember seeing DAT mined skills which gave the impression Ninja would have its own way of recovering TP, so I'd go with Pugilist and Archer.

Utsusemi tanking only worked in XI because the sub Job system was much more fluid; you could give your Paladin Utsusemi, or you could give your Ninja Provoke. Here that isn't the case.