This is based on my personal opinion, I don't know if someone already came up with this theory and this is my first time doing something like this.
We all know that Gray will kill Frosch in the near future but let me elaborate the difference in the timeline of future Rouge and the present timeline. Future Rouge's timeline is different because in that timeline Ultear didn't grow old because of future Rouge's actions in the present timeline. In my guess, Ultear would do something significant like snap some sense to gray,and would probably say something related to I too was consume in darkness in the past blah, blah , blah, don't let yourself get dragged to darkness. You get the point.
Also, some plot holes. I just want to add these.
First, in Future Rouge's timeline why didn't they destroy the Eclipse Gate or destroyed when Achnologia became active sometime after frosch's death. Considering the fact that future Lucy told of what would happen if the gate would open.
Second, Future Rouge's appearance is a dead giveaway that Tartarus failed with their plan. When I first read the Tartarus arc I remembered F. Rouge, seven years from the future and can still use MAGIC.
I apoligize for any mistakes I made in this post.