The TTS 1990 demo would be the holy grail with dark GHZ and the weird enemies. Imagine what could be dug out of that...It's supposedly lost, even SEGA have no idea where it went. They wanted to show it off years ago. We also have an approx date,
December 1990! Show off 30 years later...
Anyway moving on from that, there's some digging going on into the SA2 proto. Here's that World Ranking screen that was found way back in 2001, semi functional:
Last thing, Sonic CD 2011 has been fully decompiled...so someone ported it to Switch:
There's a Vita port now too.