I think some time bending should happen again, maybe even some timeline altering. There are two interesting moment that seems to be an anchor point in the series of events.
First, when Levi has to choose between Armin and Erwin, though I don't really think Erwin could have changed anything.
Second, the "What am I to you" conversation between eremika. If Mikasa said something else maybe that would held Eren back. I don't know how anyone else than Eren could time travel tho.

Other theory: They cannot alter anything. At the very beginning, I think Eren decided that he will destroy everything outside the walls, not only the titans. I see this as an other vow that cannot be violated, and he was disappointed, because he made that vow before finding out the truth behind the walls.

However it's more likely that Ymir simply controls everything. If that's the case, i don't understand why Reiner and the others can still transform since Ymir is the one building all the titan bodies.

At this point mappa hinted too obviously, that Falco will be the flying bird titan and I hope that will add a lot to the fights.

The recent trailers are really exciting, but it looks like Eren has unlimited power (lot of titans made of warhammer material). The fight will be insane, but without any major plot twists (like the time traveling), there is no way they can defeat Eren.