Honestly, just to be honest
I do think Fortnite has the better Crossover Roster, simply because it tackles more avenues in terms of media. Smash Has good amount of Gaming Icons, but it's just Gaming Icons. It doesn't have Marvel, DC, Movies like Alien, Star Wars, John Wick, or even other Gaming Icons like Master Chief, Kratos, and now Lara
Smash though I think handles the actual guest characters much better. While all these character appearing in Fortnite is very Epic, they all apparently use the standard moveset, and most of their unique abilities is relegated to Taunts that have no gameplay effect. It's funny to have characters like Ryu or Kratos do funny dances and use guns, but it's also said that I can't do an Hadoken in Fortnite or something like that
You can do that in Smash, And I think that's also a large reason Smash is so popular when it comes to it's guest appearances, it (Usually) does a really good job of making the guest character movesets, and other specific content. it's awesome to see
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There are also things I want to note though
-Epic Games, to my knowledge, has only Collab with Strictly Western Companies and I.P's. I think Street Fighter is the closet thing to a Japanesse I.P, but even then Street Fighter is owned by the U.S Branch of Capcom. We haven't seen Epic Collab with Japanese I.P's or Companies that much. do with that info what you want
-Remember that Sony went to that Smash Ultimate Spa tournament some time ago, and I believe it was the gaming division of Sony as well. Maybe some discussions could have started there. But yeah there is uncertainty of whether Nintendo talked to them or not, as Sony hasn't really been supportive of the Switch that much.
Now I'm sure that with Nintendo seeing that these Collabs with other Companies are what's gonna drive the hype For Smash at this point, they probably reached out to as many companies for whatever is next for Smash, probably starting with Companies that at least have, or about to have games on the System, like Epic Games, WB Games, EA, Tencent/Riot, in terms of New Companies. They might reach to Sony just because even if they don't have a presence, they do have a nice catalog of characters that could be used to sell Smash itself