I feel like Mewtwo will end up a Boss cause he was asked if his mega form will show up, not if playable plus Waluigi got tons of support but he returned as an assist so this is really making me nervous about his inclusion
What should be kept in mind is that Waluigi started as an assist trophy, so logically he would return as an assist trophy, the only example against this would be Little Mac, but even still Little Mac is the only assist trophy to be promoted, and may have had stronger popularity than Waluigi (I would say definitely stronger in the West) overall. Since Little Mac exists as the "promoted item" character I doubt they would add another character in this game in the same way.
Now then, Mewtwo started as a Playable Character, so logically he would return as a playable character or not return at all (except for perhaps in trophy form). If Mewtwo for some inexplicable reason were demoted to a boss, that would make him the first in the series to experience such a change, and I expect that Sakurai would not do so and would choose simply not to include him in the game as he has done with past characters that have been cut. Granted, Sakurai doesn't make the best decisions all the time, but I imagine he is quite aware of Mewtwo's popularity and understands how much of a slap in the face such a demotion into an "item state" for Mewtwo would be to all of his dedicated fans.
Also, as has been said, it can be easily inferred that the question that person asked about Mewtwo in Smash at the Pokemon roundtable was asking about his playable appearance. After all, what reason would he have to ask about Mewtwo, a former playable character, returning in a demoted state as an item or Boss?
As @
False Sense
Mewtwo is easily WORLDS above Waluigi in terms of popularity, in fact there is almost no other character I can think of that comes very close at all to his status. So such a demotion would make no logical sense at all.