My major hangup with Dr. Eggman honestly has very little to do with his feasibility. I'm sure they could make him work, I would love to see them try. I do however believe that Tails, Knuckles and Shadow offer similarly viable options for a second character, already have some meaningful presence in the game and have been requested about a decade longer than Eggman has. And would, in addition. presumably be easier to conceptualize and develop off an existing base.
Granted, Dr. Eggman is not a character who really
needs fan support. It's a no brainer that he is a popular and important character, but in Sonic's case I believe a new fighter would be heavily influenced by that persistent fan demand. The other three have been far more prevalent in discussion since Brawl up until Sephiroth was revealed, and I'm not sure we've had enough time since then for the tides to shift so dramatically where Eggman has become the clear crowd pleaser. Compared to say... Knuckles, who was already a Mii Costume in Smash 4 and added as an AT in Ultimate. That latter promotion tells me he must have done pretty well on the ballot, and he's been having a really great few years to boot. It's easy to get tunnel vision about who was immediately popular during the tail end of Ultimate, but Eggman's time in the sun was relatively brief.
My personal prediction would be Tails, with a mix of modest support dating back to Brawl, distinct moveset potential with his twin tails and seniority over the other two I mentioned. But Knuckles and Shadow have been just as heavily a part of the conversation if not moreso, especially Shadow since so many people were backing him as an echo fighter. And then Knuckles demonstrably for the reasons I mentioned in the last paragraph.
Which is where I will say Eggman would still make a great boss, and it would be a nice spread of content to have a new fighter from the batch I mentioned alongside a brand new boss character. If we got Sonic, Tails and/or Knuckles and then a really well fleshed out Eggman boss fight I think many people would be satisfied. But if they opted to add him as a playable character instead... well, I'd be really happy, because I like Dr. Eggman. Who doesn't?
Remember when we took the L for saying Isabelle was just a pacifist secretary?
...We?