... So. All it takes is for me to look away for one night while playing FFXIV and sleep, then a credible leak drops, huh? Guess I need to play that game more often, then.
Gotta say, I am very, very excited for this. I'm 50-50 on it being fake, but if it is, I can't even be mad, because it would be a really impressive mod, with all the details and such. Not to mention Shoryuken being nerfed, even. The last time I saw this much effort put into a fake leak was the Artsy Omni Rayman/Klonoa renders in Smash 4.
That said, just to offer a counterpoint to what some people have said: Mii Costumes don't necessarily imply fighters, sure, but we need to remember that Square Enix is the one company so far that ONLY gets Mii Costumes whenever a new fighter from their games joins the roster. So if not Geno, then a Mallow costume at least implies a Square character. And Bethesda has no content in Smash at all, meaning them getting just a Cacodemon hat would make little sense to me. So even if not Doomslayer, a costume from them could imply another Bethesda character. Could still not mean one, but I'd say the chances are low.
Geno is in my top 3, and I'd given up hope with his Spirit, but this would be huge for me. Don't really care about Slayer, but I respect Doom's legacy, so my reaction to him would be on par with Hero's.
What I'm wondering right now is: if real, what's their release order? Methinks Doomguy would be #5 and Geno would be sold separately, essentially the PP of the next season. Which is exciting either way to me.