If the Sonic franchise was owned by Nintendo, who do you think would be in Smash?
I'd guess Sonic, Knuckles, Tails, Amy, and Eggman for sure. Shadow probably too.
Shadow is the most likely out of those. He didn't become the sole AT for no reason, after all. He's the most popular after Sonic. I could see Tails or Eggman next, but honestly, all 6 of them would round it out well. Possibly 7 with Metal Sonic. Sonic being a 3rd party franchise is more or less keeping that idea low, though. While it's a fan rule that 3rd party franchises get one character, it's not actually fanwank entirely. This is more based upon Sakurai's actions and statements. He considers representation important(this is why Melee has one Kirby character specifically. It's a coincidence he couldn't make another since Dedede wouldn't be plausible with even him not making the 6 clones. That said, a clone of Kirby would've been possible if he was okay with giving more representation. Being he made Kirby, he didn't want to overrepresent his own franchise at the time. This is logical because you don't want to seem biased. He's obviously biased, don't get me wrong, but who isn't. That's not a bad thing, though), and he has called every 3rd party character a special guest outside of Snake at best. And Snake didn't come back, but of course, we don't know why(many guess Konami wouldn't play ball, but no reason to legit known). The fact they're iconic also comes to mind. One thing to suggest is that he looks at a franchise and sees "most iconic character", which could explain why the fanrule has some logic to it.
That said, Sakurai has never given more than an implication that it's unlikely, although a statement would be nice.