They already confirmed that it was a pokemon RPG for switch. I doubt they're trying to deceive us into believing it's a mainline game and it isn't. It will be a new mainline game. What that entails could be completely different though. When you think about marketing, you have to plan ahead. The Switch is going to last for many years and in order to sell multiple Pokemon games, they need to convince people that the upcoming one is new and fresh. The easiest way to do this is to limit the amount of features of the first release and then add them in subsequent releases.
One of the dominating rumors is the idea of it being a re-imagining of the Kanto games. Beyond worthless rumors this is good speculation as from a marketing standpoint it gives Gamefreak the oppurtunity to sell us a game with only 151 Pokemon. This will give them time to make it a real graphic feast with all new animations/textures maybe even new models for the pokemon. In the past we've had 3rd versions that just add a few things but you could justify that when the games were only $30-40. People are going to ask for more with a full priced console games. The real key here is that starting the Switch games off with only 151 Pokemon gives incentive for people to drop another $60 on the second or third Switch Pokemon game which would be be a new generation touting the full 900 or so Pokemon. They're not just going to give us a full featured Pokemon switch game right off the bat, they're going to give us barebones games and bleed us dry, spoonfeeding the extra features we want over the upcoming years.
