Sonic CD, with the introduction of Amy and Metal Sonic in the mid-credits scene. Perhaps we could even get Heroes representation with Metal Overlord being the final boss, and if that happens... Please, Please, PLEASE give us "What I'm Made Of..."

After that, they still have PLENTY to work with.

For starters: Adventure 1! You can't do Adventure 2 and then NOT do Adventure 1. Especially since there's still a lot of unexplored territory with Knuckles' backstory, the Echidna tribe, and their beef with Longclaw and the Owl tribe. Sonic 2 established that it has to do with the Master Emerald, and one of the few things the Knuckles show did was introduce Pachacamac. So, they planted a few seeds there. If they ever do a Knuckles Season 2, well for starters, make it GOOD and make it actually about Knuckles. They could use that to introduce Chaos and he could be the big bad after Metal Sonic for a potential 5th movie.

They could also do a Shadow movie where they explore the Black Arms and introduce Black Doom and make him the big bad of Shadow's movie.

And as MUCH as everyone HATES Sonic 06 and that God Awful story... That's the one with the most potential for the big screen. For starters, one of the very few things it had going for it was Mephiles. The writing was terrible, but the concept of him as a villain, was awesome! Same with Solaris, Iblis, and of course, Silver. And they could actually make 2 movies out of that - Sonic 6 Part I and II.

After that, they can do Unleashed. And then, they could also explore the comics, maybe introduce the Freedom Fighters and give us Sally Acorn. Or the Metal Virus arc? Maybe Ian Flynn jumps in at some point and we get characters like Tangle, Whisper, and Surge on the big screen. Maybe at some point, Eggman comes back due to alternate dimension nonsense so that you can bring him back w/o having to back up the dump truck of cash just to convince an older Jim Carrey to do it again.