Personally, I think an arc focusing on Team Flare, Xerneas/Yveltal and Mega Evolution would be cool. However, seeing as Alain is present in the intro and Wulfric is not, it seems possible such an arc will air before Ash earns his eight badge. However, hopefully the writers will do this justice. With the correct build-up, this could preoccupy a fair amount of weeks.
I'd find yet another OO/DA rip-off to be quite jarring considering we just had one at the end of last region.
Sinnoh Battle Frontier would be interesting and could take some time off our hands, but seeing how it wasn't used to promote HG/SS (which actually did have this exact Battle Frontier, too) it will most likely be used as the arc for Sinnoh Remakes.
Maybe we could travel back to Hoenn? After all, we travelled to Kanto for FR/LG. The Sinnoh Arc did drag on for a while, so it is excused as to why Johto wasn't featured, but Hoenn also required time to be killed. But the thing is.. What would they do there? Serena could try her hand at Contests, I guess? But that would leave Ash with nothing to do, and I think we're all tired of contests after May and Dawn.
Possibly, we could get something completely anime-exclusive to bide time, or we'll be getting a lot of filler episodes post-League.
Either that or they can push the league to the end of the series by making Ash fully evolve every Dragon-Type then release it whilst slowly replacing each member of his team with a Flying-Type.