Gen6 didn't do poorly, in fact quite the opposite, it's the only generation to sell more than the previous one, and not only did XY sell more than BW, but they also sold nearly as much as RS despite the lower install base of the 3DS. They are also the all time best selling 3DS games, I'd say that's FAR from doing poorly. XY were also really well received with most reviews being above the 8/10s.
I however wouldn't expect to see Kalos as a postgame area as much as I'd want to, Kalos was a REALLY detailed region, with Castles, cities, plazas, ruins, trains, skating areas, etc.
All of those would need to be made from scratch for the SM engine and that seems like it'd be a bit too much work for what would be just a post game area.
If they want to add a second region it'd sadly just be Kanto because of how easy it'd be to make, the region is so bare that 80% of it could just be made by reusing already existing SM assets, aside from a couple of buildings Kanto really has nothing unique they'd have to make from scratch.
That being said I wouldn't expect to see Kanto either, we'll probably just get some new post game areas inside the already existing Alola islands, and tbh I'd rather they add more new original content to Alola than getting an empty Kanto postgame.