Obvious statements like "This game has every Pokémon in it, making it the biggest game yet" usually always comes with press releases. Just look at WoW expansions when they're handled by the press, it usually comes with an obvious statement like "The level cap is upped to 90, giving players new heights to reach."
And of course Game Freak are working on the next game, if not next couple of games, but new Mega Evolution most likely aren't being designed this early, if anything they're probably trying to design mid-gen Mega Evolutions right now, much like how new forms of Pokémon usually are made mid-gen. Mega Evolutions probably come later in development, the designers are probably making designs for all new Pokémons right now, or they're maybe only currently designing the framework of the next game, making concepts, or brainstorming new ideas. They're early in development with the next game most definitely, I can't imagine that they're past the concept part of it since it's such a long and important process to perfect before doing anything else. Needless to say, if they're that early in development, then the Mega Evos aren't being worked on, even in an interview Masuda stated that the Kanto Megas were made when the game was actually in a playable state, since they were implemented when he realized how cool it was to obtain the Kanto starters in the game when they were just testing it. Mega Evos are likely made well into development.
I can imagine 6th Gen Megas to be the selling point of a mid-way game, like Platinum or Black 2 & White 2 that introduced new forms for a lot of legendaries.