I think the Brawl Dojo flat out said Sonic was the most requested character from fans when he was announced and put up on the site. Sakurai's normally secretive, but there's the occasional time where they'll open up and say stuff like that publicly.
He was pretty open about what characters were planned for 64 and Melee as well. It was only with Brawl did he start closing up, although with Sourcegaming's new translations it looks like he wasn't actually that secretive with Brawl, the information was just never translated.
This is how we know Mewtwo has an abusive relationship with Sakurai, since he got cut from 64, then added to Melee, then cut from Brawl, then cut again for Smash 4, but then added as DLC, except his DLC was free for most people. And it was buggy at release.
We also know Bowser was the most wanted character for Melee, and Ganondorf and Young Link were both requested a lot, which helped get them added.
So K Rool aside, who do you reckon even has a shot of winning the DLC popularity poll
I can't think of anyone outside of Isaac
Wolf, Snake, Ice Climbers, Dixie, Inkling, Ridley, Bandanna Dee, Isaac.
Isaac and Dee are basically the same situation. Popular in one region, but basically unrepresented at all in the other region.
I've noticed in Japan, the most popular characters by far are vets. And pretty much only vets. And then the Japanese start picking up characters like Dixie and Inkling that are popular all over, but also have Waddle Dee, who seems only really popular in Japan. In the US/Europe we seem to like more obscure characters and new comers over vets. Our lists usually include a bunch of indies that never even released in Japan (Shovel Knight, Shantae), random characters from dead franchises (Isaac, Sami, Wonder Red) and Wolf and Snake (although less popular than in Japan). The west really took the ballot and decided it was a dream list of their most far out ideas, while Japan seems to have stuck to characters who probably could have gotten into Smash on their own.
Ice Climbers are odd because Sakurai has outright said they won't be added and can't work no matter what, but still have a large following. Which makes sense, since they were popular vets, but... Snake's popularity started to fade once Konami started going insane, but Ice Climbers have remained steady even after several articles came out about how impossible they are.
Pichu has his followers in both regions as well and actually picked up in popularity after Roy was released, probably because people realized Pichu is the last clone not in Smash 4 and also that clone DLC was a possibility. I think it ends up coming down to "why not" for Pichu.
Squirtle and Ivysaur are not requested in any region at all from what I've seen.
I think ballot based DLC won't actually be a winner though. I think it'll be based on an idea a person or a few people suggested that really caught Sakurai's eye. Probably a character none of us will see coming, like Ryu was.