first, let me state that this is only a theory, but I feel like there is enough evidence present to say that Dr. Eggman could be joining Smash Bros as a new Sonic rep.

If you notice at the end of the Smash Bros Direct, King K. Rool's reveal followed after several screenshots of characters fighting their game's respective main villain. even looking at the new characters weve seen so far, they have been adding a good amount of new villain characters into the game, which is something fans were requesting. of the 8 characters that weve seen, 3 of them have been villains, which is not a bad ratio at all.

Then I remembered something that Sega posted about 2 years ago, which would have been right around the time development shifted between SSB4 and Ultimate. heres the article - https://mynintendonews.com/2016/06/25/sonic-the-hedgehogs-dr-eggman-explains-why-he-isnt-in-super-smash-bros/

maybe im just shooting blind, but I think there is a really good case Dr. Eggman to join Smash. i believe that in one of the next reveals we will see Eggman get announced as a new fighter and then Shadow announced as Sonic's echo fighter

Maybe he could be an echo of Bowser jr.?

Could be. They have shown all stages, so any new character would have to be from a franchise already represented in the game. Sonic is, so maybe it could happen.

I can see him as a boss maybe. 3rd party characters haven't had playable antagonists yet and I don't think they will in Ultimate. Unless Shadow breaks that mold. But even then, I think they'd not want to include 2 new Sonic reps. He'll be a boss in the "Spirits" mode possibly like Dracula and the Monster hunter dragon whoss name is escaping me at the moment...

Knuckles definitely should have made the roster. His character fits so perfect in the Smash world.

There does seem to be a focus on villains for this Smash game. Also the reveal trailers have killed of Mega Man, Mario, Luigi and maybe Dedede. Why am I mentioning this? Well the blurred out mode Sakurai talked about during the direct says "Spirits". This new mode might be a story mode where all the villains team up against the heroes or something?

There has not been any confirmation all stages have been shown. I'd be shocked if we were only getting 5 new stages total, so I highly doubt it. 

I don't think this is outside the realm of possibility. I think Shadow is quite likely as an echo of Sonic, and Robotnik as a new character makes some sense given the heroes vs villains emphasis we've seen so far.

That being said, wasn't there a list of rumours that included Dark Samus, Chrom, Simon Belmont and K Rool joining Smash? Didn't that list Tails, as well as Isabelle from Animal Crossing?

They said there are 103 stages in the game and they showed the selection screen with 103 stages. Maybe they could add more, but it would be weird to brag about there being 103 stages if there are more.

The blurred out green space on the main menu. Someone figured it out.