It's funny you should mention that it will actually be able to go beyond just a Sonic engine. I've actually been wanting to get away from just sonic and do something else, but of course with the limitations of hacking that was something too beyond my skill unless I wanted to do asm. So this means the engine will have a form of scripting of sorts so people can make their own game engines(Edit: through review of the history of this project I guess that is the case :D )
I look forward to the day Mettrix is actually finished. As mentioned above it really is much a sleeper hit, and I can't recall it making much news whenever it came out. Considering it's the most original fan-game I think I've played too, it feels like waiting for a whole new game. I know I read that the music is going to be converted from those WinGroove midi conversions to Yamaha XG synth files, but I hope that doesn't mean the current soundtrack is going to disappear as it has a certain charm to it. I'd go far as saying the somewhat ancient-sound of the conversion you get makes it feel like an old DOS game, which IMO just gives the game much more charm than anything else I've played.
Good luck on the project Stealth