I think I see Skyloft on the roster banner.

[TheCrimsonSquid]

https://imgtc.com/i/I4yEyEI.jpg

Not 100% sure whether it's Skyloft, but look behind the Mii Swordfighter and Mii Brawler. You can see a large floating mass. I think I can make out the Goddess Statue.

EDIT: Actually, I think this may be the world Spirits takes place in.

 

[Yosher]

I guess that could be Skyloft, but.. what does it matter? Skyloft confirmation? Skyloft is already confirmed, in case you didn't know. I dunno if this warranted creating a new topic.

 

[TheTrueBrawler]

I can't exactly tell, but it is a stage confirmed for the game, so I wouldn't put it past me. It could still be Temple though.

 

[TheCrimsonSquid]

Actually upon studying this closer, I think that it looks less like Skyloft and more like an entire world split into different sections, and divided in half from top to bottom.

This COULD be where Spirits takes place.

 

[TheCrimsonSquid]

Alright, the mystery has been solved.

https://www.ssbwiki.com/images/8/86/SSBU-Battlefield.png

It's the floating island in the background and to the left. Matches up perfectly.