A Metal Sonic (Sonic CD version) shows up, followed by multiple of them, but Amy also shows up and one shots them. So we know Amy is coming for the next movie. And post credits, Shadow is shown to have survived. Eggman may be killed off once and for all though as Jim Carrey doesn't want to do it anymore? He only did this film because money apparently. I need to research that.
I had hoped Metal Sonic appearing would be the end of it and we'd get a nice Sonic 4 movie with Metal Sonic as an antagonist, but the fact Amy shows up to dust a bunch of them with her hammer sort of lessens the threat immediately. The Metal Sonic appeared nearly similar to how a terminator would appear in the movie (time travelled/teleported) so my initial feeling was we'd end it at that, but I didn't expect to see Amy. Now that Amy is shown to be capable/strong, or that the Metal Sonics are just weak, it's all a waiting game now until the next movie.
I really like Metal Sonic because I consider it a rival character, even though that never really happened and Shadow eventually took that spot, more or less. Its appearance in Sonic Adventure 1 was cool but was more of a cameo/reference. It's why I like the Sonic animated OVA movie so much back in 1999 because it was built by Eggman to combat Sonic and it had some somewhat cool fight scenes!