I'm honestly on the fence about whether Jim Carrey should come back.
On one hand, he killed it in this movie and it's hard to see how he could top his performance here, especially if he comes back and just does another typical "try to take over the world" plot. The way Sonic 3 ended was a good conclusion to Robotnik's character arc, and it might be better to go out on top. Plus bringing him back immediately would sort of cheapen the end of this movie.
One the other hand, it's hard to see someone like Metal Sonic carrying a movie. And it's hard to justify Metal Sonic being around without Robotnik, since Robotnik created him. I guess they could say Robotnik created Metal before he died but that would be a little hard to buy.
Guess it comes down to whether the writers have something more interesting for him to do than just send Metal Sonics at the heroes.