There's another possibility than Frosh's death. Let's see what hints Mashima gave us:
-The team that was going to take on Avatar by themselves was composed of Minerva, Rogue, and Frosh
-The team that will try to take down Avatar is composed of Lucy, Natsu and Happy
-Frosh's death led to the future Rogue that fell into darkness
-Happy and Frosh are equally dumb and cute and like each other a lot, and we can also assimilate Carla with Lector who are both the more intelligent Exceed of the pair.
All of these made me think that bad Rogue's future is not going to happen, but instead Happy might be killed by Gray which will cause Natsu to fall into darkness, awakening E.N.D. If that's so, I'll ragequit because NO ONE should kill Happy. Never.