Those who read X-Men comics know just how expansive X-Men universe really is. Days of Future Past is certainly an ambitious project (here's hoping Ian Mckellen makes an appearance as future Magneto), if they manage to pull that off they may as well introduce Apocalypse (one can dream), the same way Avengers did with Thanos, and there's no Apocalypse without Cable. That's why I feel there's no need to connect X-Men with other projects Marvel has going on right now as X-Men universe can stand on it's own perfectly fine.

Confession time! I don't really like Fantastic Four, in almost every media they seem/are kinda dull.

As for Thanos, seeing as Infinity Gauntlet is in Odin's treasure room he'll make an appearance in Thor: The Dark World, I'm sure of it. Tesseract or Cosmic Cube if you will is also there so he might snatch that as well. One thing is certain, he'll make one hell of a villain.