A thought occurred to me a few days ago. I wonder if the possible/eventual Persona 5 Arena will just stick with Joker rather than Ren Amamiya as was established by the anime & other spin-off games (plus what ASW uses on Twitter). Now that Smash beat Arc System Works to the punch, most will know him as Joker. The part that has me curious is that if ASW sticks with the Ren name, it may cause confusion for those who know Ren/Joker from Smash.
As for my prediction, I feel like it'll depend on how they name the other Phantom Theives in P5A/P5U. If the likes of Ryuji & Ann go by Skull & Panther (like they're referred to for their Funko POP figures), ASW will probably follow Sakurai's lead & just refer to Ren as Joker. On the flip side, if they use the full Ryuji & Ann names, maybe they'll go with Ren.
Likewise, after what happened with Roy, I'd imagine that Sakurai wouldn't want to lock down any ideas for Byleth, Edelgard, or anyone else from Three Houses until the game is just about to go gold. At least with Corrin, Fire Emblem Fates had been out in Japan for months prior to Sakurai pinning him down to be included in Smash. In this case, everyone was nailed down as of November 6th, 2018. From then, Three Houses would have been over 8 months away from release.