Considering that XY had a lot of Gen I nostalgia (and ORAS obviously has Gen III), I'd like to see the next Kalos game(s) give some emphasis to the Johto starters and legendaries. As for the others? I can see at least some of Gen IV's being in Hoenn, due to Regigigas (Heatran could also fit into a post-game area of Mt. Chimney). I also doubt they would want to dump both dragon-trios into one game, so maybe we could see something like:
ORAS:
- Regirock, Regice, Registeel, Latias, Latios, Groudon, Kyogre, Rayquaza (no wai!)
- Azelf, Uxie, Mesprit, Dialga, Palkia, Giratina, Regigigas, Heatran, Cresselia
Next Kalos Game(s):
- Raikou, Entei, Suicune, Lugia, Ho-Oh
- Cobalion, Terrakion, Virizion, Tornadus, Thundurus, Landorus, Reshiram, Zekrom, Kyurem
- Xerneas, Yveltal, Zygarde
Which makes a total of 17 legendaries for each release.
I think it would make more sense to have all the Johto legends plus maybe the dragon trio in ORAS, with the pixies, heatran and cressela possibly in the next Kalos game seeing how they were all available in BW2 which was fairly recently.