Yeah, it's really getting hard to predict at this point. There's any number of directions they could go in, some more likely than others.
Eclipse/SM2- Like
@SkyeVictini said, Game Freak has made a point in not being too predictable recently, they made BW2 instead of Gray and SM instead of Z. If we're getting this at all, it'll be in some unique and unexpected way.
XY2- Zygarde is off the table as a mascot since he's getting focus in SM, but what about Xerneas and Yveltal? They don't have Megas or Primals yet, so they could potentially get new forms of some sort and they could connect them to SM and whatever's happening with Zygarde.
Hoenn revisit- Groudon, Kyogre, and Rayquaza all have Megas/Primals, and recently they've been using new forms and Megas as mascots for revisit games (Giratina got Origin Form for Platinum, Kyurem got Black/White Kyurem for BW2, Groudon and Kyogre got Primal forms for ORAS). The fact that they gave Rayquaza a Mega in ORAS implies that they probably don't plan on returning to Hoenn anytime soon
Kanto revisit- RBY being released on VC decreases the likelihood of this happening, but there is an argument to be made for a full, modern Kanto game. Another Kanto remake probably wouldn't be interesting to them at this point, but they could potentially do a sequel and bundle a copy of the VC game for players that want the original story for reference. They might not want to mess with Kanto too much though because of nostalgia, they could really upset Gen 1 fans.
DP remakes- This could happen, but they don't seem to like bringing back old regions until they're no longer backwards compatible. This probably won't happen until NX when DS games are no longer playable.
Multiple regions in one game- I don't think they plan on doing this anymore. Multiple regions tends to cause problems with game pacing and content, and they'd probably rather focusing on fleshing out one region than cramming in two shallow ones.
8th gen- We're just starting a new generation, so we probably won't immediately move on to the next gen so soon. 8th gen would probably be 2018/2019.
So if I had to guess, probably XY2 but there's no telling anymore.