I think just about everyone was hoping Megas would continue, and I was a little shocked they didn't continue after how big of a success they were (not to mention how much they hyped them going in the 6th Gen).
When Alolan Forms were first announced, I really thought they would be doing them for each and every Kanto Pokemon. But no, we got like ten. Would've been amazing to change all the types and abilities of the original 151 and give them each a fresh alternative, especially when the Alola Dex was so very small. They could've then flipped that and given each of the Alolan Pokemon a Kantoian Form. But that was just my thought.
I think most fans would love to see Shadow Pokemon. Seen in Colosseum, Pokken and now GO. Can't believe they haven't brought them to the Mainstream Games. Distortion Forms would've been cool for the 4th Gen Remakes, especially since Giratina is the god of this alternative universe, but that's obviously not coming.