An ex-member of STI, Craig Sitt has found screenshots of a cancelled Sonic game proposal. It was to be a spin-off featuring Tails, named Treasure Tails. The game seems to be an isometric puzzle platformer taking place in a South American-like temple. The stills weren't created by Craig, rather Bill Dunn, who pitched the concept in February 1993. Please note that these are only concept images for a pitch, there most likely isn't any sort of prototype.
Now, would this proposed game happen to have any relationship to Aspect's Tail Adventure, which came out 2 years later? Maybe the concepts interested SEGA, but not enough to get this the okay, to be open to a spinoff Tails game. Maybe Aspect got the idea to make a game based around Tails from hearing about this proposal, if they did, or maybe it's just mere coincidence. Just my thoughts while I wear my tinfoil hat. Nice find anyhow by Craig.