Sonic Mania Plus confirmed, it's a physical release of the game, with a reversible Genesis-style case sleeve, a 32-page artbook, the playable characters Mighty the Armadillo and Ray the Flying Squirrel added, and an "Encore Mode." I believe they teased additional features but didn't reveal them yet. I'm not sure if current owners get a free upgrade, I saw chat talking about something being "free" but I had to quickly take care of something and missed that announcement if it happened.
Also they announced a series of animated shorts called Sonic Mania Adventures, coming later this year. They'll be animated by the animator who did Sonic Mania's opening animation (wow this sentence is redundant), there'll be five of them, and they apparently take place just after Classic Sonic returns to his world at the end of Sonic Forces. They showed a small teaser that's currently unfinished but even the teaser looked pretty good for what's to come.
EDIT: Here's the sneak peek.