The whole Jane issue surrounding Joker I find incredibly intriguing, if for nothing more than why would she have been there in the first place? Prior to The Royal's reveal, I was completely with people on the idea that we'd get a Female MC in another version of Persona 5 thanks to the return of Persona 3's MC in Q2 and just how many people seem to generally clamor for such an idea. I still think it's a great idea that Atlus should have gone for since it potentially changes so much of the game and relationships up, but alas, here we are. When they revealed Kasumi as just another member of the Phantom Thieves (Well, probably not quite just another member, but certainly not a Female MC level of new character), I was extremely confused as to how her new role could ever be considered for an Alt for Joker. I mean, you could intentionally do a lot of things with Kasumi to make her a very big part of Royal's narrative, especially given that the game will feature a brand new trimester. But at the same time, I remember Marie from Golden, and despite being one of the few to very much love her, Atlus did not do a particularly great job of integrating their new character in Golden.
While I suspect Kasumi will take many of the lessons learned from Marie and Golden to become a properly fleshed out and integrated character that directly impacts gameplay and story, we still don't know exactly how much she'll actually tie into the game yet. I don't know where she'd be introduced in the story quite yet, but one would assume early on given that she seems to function very much as one of the main, new features in The Royal. If she was considered for Smash, I'd almost wager that she has to play a substantial role in the game. It's the only way the Jane file in the Joker data really makes any sense. As to what her role will be? I have no idea. I'm still working on Persona 5 right now and while I'm aware that Golden added a lot of stuff, The Royal really seems to be going above and beyond the QoL improvements and additions that game did.
Will be see Kasumi in Smash though? That's a much more difficult question for me to answer. If they got as close to completion as people think that they did, I don't see any reason why they couldn't just drop her at a later point. Costume DLC for existing characters has never been a thing, but I don't see any reason it couldn't be and would be the simplest way to make sure that the work done on her actually went somewhere. Then again, she might also be retooled into an Echo for release later down the road. Sakurai seems to be an absolutely massive fan of Persona 5 in general, and I'd imagine he'd be ecstatic for not only The Royal, but potentially the opportunity to play a bigger role in that game's success and future. I'm sure that Atlus has almost no reason to turn down any opportunities to further the Persona franchise, so they'd likely be incredibly happy to work out a deal with Sakurai to release an Echo closer to the launch of The Royal. Though again, we haven't seen as much of Kasumi as I would like to really speculate on how she'd work as an Echo. I don't think the Persona would be too much of an issue since Arsene doesn't directly interact with players anyway, and there's no reason for her to appear too differently from Joker, so I'll say for now that she could work as one.
I think it just comes down to a question of, What is Kasumi's role in Persona 5 The Royal, and does it warrant Kasumi as playable in Smash? If she's not as relevant to the story and The Royal as we think, I'd definitely get the sense that they were adding an Echo just to be adding an Echo. Which isn't really a problem, but is just kind of odd when there are still several options for Echo fighters available. But if she plays a bigger role in the game, then I think there's a much more compelling argument to see her inclusion in Ultimate as part of the Persona 5 name.
I've thought that there was a good chance for there to be some extra Echoes that show up at some point. They seem like such an easy win for Sakurai and people are pretty much foaming at the mouth to see more content in Ultimate. I do think when the Fighter's Pass concludes, that will be the end of major content updates for the game though, so it's about a 7 month window til then. If we're going to get her as an Echo or Echoes period, I would suspect them in that time period. Probably near the end honestly as I could see a Smash Bros. Final Direct like we got with Smash 4. I can't say I'm super keen on her chances for getting in, but I more than welcome any additional Persona representation in the game, and she'd be a really cool Echo to have. I think we'll really understand more about her chances once we know more about her haha.