One thing I figured would be interesting discourse is how Sonic and the possiblility of it getting a second rep has evolved over time. Sonic, at least in terms of smash speculation, was always looked at a bit specially and there was some desire for a second rep. While it was never particularly a mainstream idea, it still was around. Starting in Smash Ultimate, with the idea of echo fighters, Shadow really picked up a lot of steam and to be honest, I can easily see him becoming an echo next time. But he did not make the cut. Sonic reps did get a huge spike in discussion when Sephiroth came along. While Sonic as a series was entertained in the past lightly after echo fighters were a thing, now people were considering if Sonic could get a unique character. Due perhaps to a real glut of iconic villains to push left, Eggman wound up getting a huge groundswell of support. Tails also got some support with his possibility being more likely in the future.
Fast forward to now, and it honestly is interesting that people seem to have this mentality now of when rather than if when it comes to a second Sonic character. While we could get into a tedious discourse on why this is the case and Sonic seems to get special treatment in that regard compared when you factor in other third parties, let's ignore that and focus on the potential candidates. In my opinion, these four are the most realistic candidates: Eggman, Tails, Knuckles, and Shadow.
Ironically, I would say Shadow is probably the most likely, but that is mainly due to how easy it would be to make him an echo fighter of Sonic and how widely popular the idea is. Sure, some people would prefer a unique Shadow, but to be honest you can easily make Shadow an echo or slightly different clone and you can still have a wildly popular newcomer pick without much of the effort. In terms of him being unique, I do not see it being that likely especially compared to the rest of the classic crew.
In terms of unique newcomers, I think it is between Tails and Eggman. Tails you can make as kind of a semi clone like Toon Link, which would lessen the workload. Aside from that, there are strong arguments to put both of them in. Both are pretty much the only true recurring Sonic characters that show up all the time outside of Sonic. Both are widely iconic characters in their own right. I think honestly it is not infeasible to end up getting both in Smash someday. If I had to say which one I would prefer, it would be Eggman, but that is mainly due to Robitnik being one of the last huge iconic gaming villains we have left to add into Smash. Also, a lot of what I said about Tails and how he is recurring and can be a Sonic semi clone can be said about Knuckles, but honestly I think that Knuckles is kind of kneecapped a bit due to not being as consistent of a face in Sonic over the last fifteen years or so compared to Tails.
Gun to my head, if you asked me what Sonic's representation would look like in the next smash, I would say at the bare minimum we would probably see Shadow come in as an echo fighter. I could see Tails and/or Eggman showing up as well, but I am not nearly as confident as us getting a Shadow echo fighter.