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