First we should look at everything that came from the trailer and content itself:
"Shantae x Blaster Master Zero" = "Super Smash Bros x ____" (I also noticed that the actual games are first, and the games crossing over into them are second)
"A new transformation approaches" = "A new challenger approaches"
And then there's a lot of evidence that follows the theme of "Everyone" which is also the theme of Ultimate.
"Jammies Mode is HERE" = "Everyone is HERE"
"Free for Everyone, regardless of version, backer or not" = Follows the theme of bringing everyone in
"Ultimate Slumber party", in which she wants everyone, even her enemies = The Ultimate Smash Game
Shantae says something to Risky along the lines of "My Ultimate Slumber party wouldn't be Ultimate without you". This could be a metaphor for Sakurai and someone who was particularly hard to get back, like Snake, considering how both Konami and Risky both came off as reluctant to join, since Konami was completely absent from SSB4 and Risky is the main villain of her series and requires 2 boss fights to cave in(?).
There's also something in the ending involving Mimic, but i don't really understand why people are using that as evidence. (Can someone tell me?)
Regardless, there are too many things that align for many of them to be coincidences. Maybe if it were one or two things, but six (at least)? They're definitely playing at something.