Hi everyone! I fairly recently picked up Persona 5 (roughly a month ago) and it has easily become my favorite game of all time, aside from Smash. I regularly post in the general speculation thread, but I'm going to bring some of my reasoning for why Joker is possible to this thread, and I think people may be sleeping on him.
1. Atlus has been a long-time supporter of Nintendo consoles. Atlus has been publishing games on Nintendo consoles since as early as 1987.
2. Atlus was granted the ability to do a crossover with a Nintendo IP with Tokyo Mirage Sessions #FE, an honor many companies have not been allowed.
3. Sakurai played Persona 5 as of 2016, which is when the roster would've been decided, and he gave the game heavy praise in basically all respects.
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https://personacentral.com/super-smash-bros-series-creator-discusses-persona-5s-ui/)
4. "Why would they not use a Shin Megami Tensei character since those games are released on Nintendo consoles?" This is an interesting question, as it would refer to a mutual benefit in the sales of both Nintendo and Atlus games. With the release of Q2 coming, though, that is no longer an issue. Also, see Cloud. He doesn't have any notable appearances on Nintendo consoles, but he was the character they went with. If Atlus is approached about getting a character, they're going to choose the focus of their marketing. Right now, that would be Joker and Persona 5.
5. Atlus advertises the crap out of Persona 5, even now that it's been a year and a half since the games Japanese release.
6. Who is competing with the possibility of Joker? Many people agree that there aren't many shoo-in characters for Smash Switch. So this is a perfect opportunity for Joker to sneak in. He struggles by having the third-party tag, but based on circumstances like TMS#FE, its obvious that Nintendo and Atlus have a positive, working relationship.
7. Perhaps not overly important, but the game has been critically acclaimed, some even saying it "sets the bar for JRPGs going forward".
I think Joker is a third-party character who should NOT be slept on for Smash Switch. I do believe he's still what would be considered a "long-shot", but aren't most third parties before confirmation? Nobody imagined Snake, Ryu, Cloud, or Bayo would be in the game, but look where we are now. Don't think your hope is misplaced if you're a supporter of the Phantom Thief.