I don't think SMT overlaps with Persona at all in terms of fanbase. I mean, there's obviously an overlap for multiple and obvious reasons but Persona 3, 4 and 5 have essentially their own fanbase for each game. Especially 5. Then there's a more overall Persona fanbase which is still more dedicated to the newer entries.
And I don't agree much with the point when anyone could argue we got picks that cater to audiences that already got a piece of cake in base game/Fighters Pass 1. Heck you can even find people that thinks Hero appeals to the Persona audience as much as Joker only because both series are JRPGs despite of being drastically different.
I'd say a Shin Megami Tensei character would appeal to mostly a new audience and at the same time, would bring more attention towards the series. A lot of Persona fans have heard about SMT at some point and probably are familiar with the notion that Persona and SMT are related, but never gave the series a try for multiple reasons. A Smash character could push them to do that. This would also apply to other people, not only Persona fans, of course.
That is a thing Nintendo and the third parties seek. Nintendo has seemingly funded SMTV, which is an exclusive game and the first Atlus game to have a worlwide release, which is a HUGE deal for Atlus. And I'm pretty sure part of that is thanks to Nintendo. So Nintendo would definitely want the game to be a success and I'm sure they're gonna push it a lot when the time comes.
So I could see them pushing for a SMT character in a similar way to what happened with Bayonetta (a game that Nintendo has interest to push because of exclusivity reasons) or Dragon Quest (a historic series with strong ties to Nintendo).
And in this case, these 2 reasons apply to Shin Megami Tensei, which is something that not a lot of franchises can say in regards to Smash. Yeah, the series is still kinda niche and all but sometimes I feel that when it comes to SMT in Smash people are very quick in dismissing the possibility when the franchise has some arguments in it's favour that a lot of people use for other IPs but for some reason it doesn't seem to be as valid with Shin Megami Tensei.
2017 and 19 were really strong years, but 18 and 20 were kinda weak with only one or two big games. If the pattern continues, this will be a good year.
I hope so, and if that's going to be the case we should see more about that soon, with the E3 Direct as the final stop. I'm sure we'll see Breath of the Wild 2 there. I believe we could see Bayonetta 3 or something Metroid related there as well, I think it's already time for that. There's also the rumoured FE remake and whatever Monolith Soft is working on.