I brought this up a month or so back, but it is really telling that the only 2 characters that have been DLC and AREN'T the main protagonist have had other circumstances surrounding them. First we have Mewtwo, who was not only the first DLC character but also a returning character. Then we have Plant, who definitely has enough things pointing to it being a base game character that couldn't be finished/got held off (the guidance and the fact that it was being playtested around September 2018 both indicate this). And yeah, they pretty much explained that they consider Min Min a main protagonist as well. We know characters like K. Rool, Ridley, and the like did ridiculously well on the ballot; but they waited until the next game to add them in (they opted to add in Cloud, Bayo, and Corrin instead who are all the main protag for their respective games).
This obviously doesn't deconfirm secondary or spin-off characters, but it does definitely showcase some of Nintendo/Sakurai's trends for favoring certain characters. It's fun to note that Sakurai's backup pick for a DQ character was Slime, and not someone like Erik from DQ 11 or Bianca from DQ 5 (who is really popular). The slime is the mascot of the series obviously, but a fun coincidence is that it has it's own series of games with it/them as the main character (though the last release was like in 2011).
I think characters like Waluigi, Geno, Dee, Toad, Dixie could still happen but it's just another hurdle that they have to overcome.