To completely prove your point, I will present my theory:
I am absolutely certain that it’s some sort of Subspace/story mode, and I have evidence, in the form of Rathalos. Rathalos is, as they stated, both a boss and an assist trophy. The stage in which Rathalos is a boss is this one here (my apologies for the shitty YouTube screencaps):
It’s a pretty cool stage, but what’s most notable about it is that it is not in the stage select menu that they showed us (which, I might add, is supposed to have every stage from the get-go)
They told us there were 103 stages in total, and there are exactly 103 in this menu, but there is no Rathalos/Monster Hunter stage.
In conclusion, Rathalos is a boss in some sort of story mode, and the only person that can convince me otherwise is Sakurai himself
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