So this is what the mode looks like depixelated according to most of the community. I've seen different versions of it, but this is the general idea: an ember above the word "Spirits".
The question is, what is exactly spirits?
Some people speculate that it is how you unlock the characters: the villains kill them, and you get their spirits back. But I doubt that, since Mario has already been killed in a trailer already, and he's in the starting roster. But maybe there is a way around that.
They also say this might actually be a new story mode. If it is true, I'd say that I don't really expect anything in the scale of the Subspace Emissary, but probably bigger than Melee adventure mode. But, this is the big rumor about it. With the Heroes vs Villains theme going on here, I think it's clear that it could hint at that being a focus of this mode. And everyone getting killed meshes pretty well with a story mode: a quest to bring back the fallen characters, and probably recruiting the villains to maybe defeat one common enemy?
Although, "Spirits" could just be a codename, since Sakurai surely knows that fans will use everything they have in their arsenal of softwares to try and decrypt this pixelated mess. And they did depixelate it. But if this truly is Spirits mode, then... what could it be?