My speculations of why Ultear is brought back.
She still might has unfinished business with Rogue, she might have solutions regarding Rogue's mysterious shadow that turned into the Rogue six years from the future.
At worst, she might think of killing Rogue again.
Gray still might have something inside him that killed Rogue's cat and the reason Rogue made into what it was from the six years from the future in the first place.
Maybe this time she might consider killing Gray to change Rogue's fate?
Since Ultear and Jellal are working together to beat Zeref, E.N.D. that is Etherious Natus Dragneel might stand in their way.
And if these two once learned about E.N.D.'s true identity, will they consider Natsu an enemy?
Gray wants to destroy that book, so working with Ultear to become Natsu's enemy?
Will they still consider of defeating Zeref?
Seriously Natsu was once the master of Tartaros from the very beginning, and now.
Gray is something to be consider as threat currently for some reason.
And Rogue's future, I think it might turn out differently.
The worst thing is that Acnologia and Zeref might spread these "truth" in this middle of chaos to bring heavy despair to everyone.