Well according to
SKX31
, Nintendo does seem to have some sort of current plan with ARMS, and while I don't think Min Min really works as a promotional pick due to the age of the game, there might be something we don't currently know about coming down the line.
Contrasted with the Kirby and Donkey Kong series, and it's likely their new games are a ways off with the earliest date for the next mainline Kirby game being somewhere in 2022, and a third game in the Donkey Kong Country Returns series being in question since Retro Studios is currently working on Metroid Prime 4.
As far as support for the characters goes, to say that they're about as popular as an ARMS character sounds about right to me since I never really saw much support for any of them for this DLC cycle. Granted, this place is my only frame of reference but...
I personally think they're long shots, heck, Min Min herself was a long shot. Rex & Pyra/Mythra as well as they're kinda in a similar boat, albeit one with a Xenoblade Chronicles: Definitive Edition sized hole in it.