I'm sorry to put a downer on people saying that there will be new Shellos and Gastrodon forms, but unless there is a time capsule trading system its not going to happen. Who says we're even going to be able to catch them in the game? For future generations and lands we might see new forms of the two, but probably not in this game.
This is right. I know I wrote something above about Shellos and Gastrodon but that was in case the developers really wanted to put them in. Shellos and Gastrodon are like Unown, they would need new sprites if anyone wants to take them to Sinnoh. And getting back to old sprites is meaningless. Right?
If there's a Bill's Time Machine, or anything that does not allow trading new stuff to DPPt, then that makes space for a lot of new things