I just had a thought: With those who may have done well on the ballot but did not get in as playable characters in Smash Bros. Ultimate? (Say Isaac, Bandana Dee, Waluigi, Dixie Kong, Crash Bandicoot and maybe Geno), I think that Sakurai noticed how popular they were, but since making all playable characters who were playable in any Smash game before a priority, it was difficult to bring them back all at once. Some of those who did not get in as playable in the base game were made into Assist Trophies, some were added as DLC, and others may have been put aside for other characters for promotion reasons (like maybe Sephiroth over Geno, maybe to promote the Final Fantasy VII remake games, but this is just a guess), as some DLC characters were added for promotion reasons.
BUT anyway, my point is that if Sakurai is still using the ballot for the next game, and with how some characters did well to get into Ultimate in some way, maybe they will have a higher chance of becoming playable in the next Smash Bros. game. I think that without focusing on bringing everyone back like in Ultimate, (Sakurai did say it may not happen again, did he hot), there may be a higher chance of those characters making it into the next Smash Bros. game.