I can't really speak on Xenogears/Xenosaga as I haven't really played those and haven't followed them.
SMT though, I have a few thoughts:
I think a Nocturne remake/remaster/port is inevitable. Atlus has expressed interest it within the last year, it would be a great way to build hype for SMTV (releasing it on Switch) and it's the only mainline game that hasn't been released on any other platform than it's original platform, other than IV and IV Apocalypse (NINE does not count). I'm honestly expecting it, knowing Atlus it just makes perfect sense. Even if it ends up coming after SMTV, I would still think it will happen. They could easily get mileage out of the tech they are using for SMTV to do a proper Nocturne remake.
I, II, and if... I think will likely get ported to something else again some day, but it's hard to say what, and even less likely it gets released in English. Those games are currently available on smartphones in Japan (both Android and iOS), but SMTI released in English on iOS, and now doesn't even work on modern versions of iOS. It also flopped and they never bothered to update and fix it, and it seemed like the last time Atlus USA was interested in localizing that game, II or if... I think there's sort of an understanding there that Atlus USA knows it's fans will just play fan translations and that it's more lucrative for them to let sleeping dragons lie. I hope Atlus USA changes their minds one day and localizes it the next time it gets ported, but I doubt it.
Strange Journey just released on 3DS about a year ago in the west, more than that in Japan. I wouldn't count on seeing anything about that one for a very long time.
IV and IV Apocalypse I wouldn't expect to see anything on, but they were both moderately successful (I believe IV moreso than Apocalypse) so I think it wouldn't be unreasonable to see them ported/remade some day. I would guess they might be remade, just because their look for the 3DS of 3D mixed with 2D sprites wouldn't exactly age well. Handhelds have moved to stuff like Switch, and I don't think the style of visuals IV and IV:A used would quite hold up there. However I wouldn't say no to straight ports if they did them.
No one cares about NINE, that one can stay buried.
For spinoffs, Soul Hackers was released on 3DS six years ago. Devil Summoner had a remake on PSP that never came to the west (not the Raidou games, the original Devil Summoner). The Raidou games might get something some day, but they are a bit lesser known. I would like to see a new game in the Devil Summoner franchise though, involving the Kuzonoha clan, Rei Reiho, Dr. Victor, the Gouma-Den, etc. There's a lot that can be done there that I think has yet to be explored.
Digital Devil Saga I would love to see come back in some form, even if it's just remakes/ports to another platform.
I don't really have a particular desire to see another Devil Survivor, as I think I had my fill of it after 2 Record Breaker, but I could see a Devil Survivor 3 happening.
