I'm sorry, I wasn't meaning to act overly defensive. But if you have a crossover that doesn't actually cross over characters, you are missing the point of what a crossover is, and what fans want to see in one.
If you're just crossing gameplay styles, it's honestly a lot easier to just make a new franchise (you've got all new characters anyway). I mean, then you wouldn't even have to worry about the royalty sharing or any of the other things.
The point of a crossover is to put familiar faces together. That's the entire point of it. That's how every crossover has ever worked, from movies, games, television, or books.
There's also the marketing concern. Even if they meant to just market the series with the faces, once they made the trailer, they kind of have no choice.
Imagine if there was a Smash 4 teaser when the game was first announced featuring Mewtwo, Ridley, Shulk, and K. Rool. And then none of them were playable? In addition to royally upsetting the fanbase, there are serious legal issues there.
So, yes, they will be playable.
Also, we know it's not going to be a massive gameplay crossover, because Intelligent Systems confirmed they were working on it as a more traditional RPG style more akin to ATLUS's games (ATLUS is in the lead on the project), rather than a tactical one.