Hi, new person here.
Like many, I was curious about the revival of Ultear, despite her sacrifice at the Grand Magic Games. But I do not think she's going to be back permanently, rather, she called that pocket of space the "Gaol of Time," or "Prison of Time." This is a world that exists outside of time, isolated from the place we know of.
If anything, Ultear casting the grand time spell to turn the world back one second costed her youth, or her time, imprisoning it in another area away from usage, because it has been spent.
I'm willing to bet that this is Ultear in her youth, the "time being" that was not able to live. And by activating her time seal magic, that Spriggan 12 person either lowered the prison that held Ultear's time, or brought the prison to their battleground, thus releasing Ultear to fight. (I mean, she was wearing the same outfit she had on from the Grand Magic Games when she casted the spell).
That and Mashima might cheat or something, and have a huge battle that will result in the "time" of the Spriggan 12 member switch places with Ultear, freeing Ultear's time from the prison, because that's how he would roll.
Also, where the heck are the remains of the Oracion Seis?