ARMS is weird huh? Because like, in the context of ARMS itself you have characters like Helix, Dr. Coyle and Max Brass who are completely unique in every way and don't fight anything like Min Min. But put these characters in Smash, and the core mechanic of ARMS suddenly makes them feel a bit more redundant when Min Min is already covering the Dhalsim-esque zoner brawler archetype. Since Min Min is built to showcase what's unique about ARMS' general gameplay / the sport itself, her specials and basic punches are shared across essentially the whole cast.
I think there's plenty that can be done to differentiate them, they would at least fight quite differently from Min Min's martial artistry - but if they follow the model of ARMS as it is currently represented in Smash, yeah they would share a handful of key moves. I think you'd be best off adding a different weight class like Max Brass so you can at least play around with more of a grappler styled approach... as hellish as that sounds to deal with. Otherwise someone like Spring Man could probably be a semiclone as previously stated, and of course they could all use unique ARMS to varying effect.
Really feels like the question is moreso "will Min Min come back" rather than "will ARMS get more characters" though, unless they surprise us with a proper followup for the next console. I hope they do, I think the series has potential and a fantastic cast that I would like to see at least given one more go. Just for the record I do think Min Min will return to Smash, because however you feel about its legacy ARMS is the only Switch IP on the roster as it stands. And we don't have a lot of precedent for outright cutting off first party series anyway, so that's enough in my eyes.
I do think there's definitely some things you could do, but I feel like a lot of them work better as unique aspects specifically in a setup like ARMS has, I guess is a good way to put it (like Helix's stretch works better in an arena fighter where he can dodge fists, rather than on a 2D plane, for example). If the character in question works like Min Min does off of that framework, then that's jab, forward tilt, forward smash, neutral special, side special, down special, and grab, and while that obviously still leaves a lot of room for changeups in the other tilts, smash attacks, recovery, and throws, I guess it just kinda feels odd to me personally because of how much Min Min already embodies ARMS' gameplay on the whole? Definitely something that could happen maybe if a new game came out during the planning phase, though I wouldn't expect it myself.
I do think going to a different weight class would be the best shot at making something work for that though, I'll agree.
I also think Min Min is going to come back, basically for the same reasons you listed. I think the only time an entire first party series was cut was the Ice Climbers in 4, and that was because they physically couldn't function properly on Smash 3DS, rather than just being axed.
