Earlier this week, Sonic character designer Naoto Ohshima shared some illustrations of an action game he was working on in 1989 that evolved into Sonic the Hedgehog.
According to the character artist, the project, to which these images belonged, was going to be called 'Twin Stars' and would have followed twin brothers who must protect the Dream World from the boss of the Nightmare World, "Thirteen" (pictured as a giant grinning figure in the sky). The project, however, was never fully approved, with various concepts winding up in different iterations of the first Sonic the Hedgehog game during its development.
For instance, the early concept art of Sonic shows Nightmare still lingering in the background, while another rock-like enemy also seems to appear in both. The location in one of the Twin Stars' images also looks notably reminiscent of Green Hill Zone, with its tropical palm trees and exaggerated loop-de-loops.
As Ohshima states on Twitter, "What remained of the Twin Stars project was smooth terrain, loops, and [running fast]." After settling on those attributes, that was when he started developing characters for what became the internal mascot competition at Sega, which led to Sonic. You can find some more documents from Twin Stars posted below, by one of Ohshima's former colleagues.
Obviously, to fans of Sonic Team's other games, the brief synopsis given above will sound extremely similar to another project that Ohshima worked on in the mid '90s: NiGHTS into Dreams.
That game followed two siblings and a creature called Nights battling against the evil ruler of Nightmare, Wizeman the Wicked. But in replies, Ohshima called Nights "irrelevant" to Twin Stars and said he was "thinking of a completely different world and characters" when designing Nights.