In the 90's Sonic has suffered many games that had been scrapped. Let's take a look.
In 1995 when the Sega saturn launched. Sega needed a killer app for the system. It was ment to compete with Nintendo's Super Mario 64 coming and Sony's Crash. The engine in the game used was going to be the NiGHTS engine. They used a fish eye engine instead. Many People at Sega of america wanted to get this game finished working at 16-hour shifts. The game was set to hit stores during the 1996 holiday season. The levels were only 20% done. Chris Senn was the person who led the project. Sonic X-Treme was announced in May, 1996, at E3, where a playable demo and video were available. There on Youtube just search it, also look at for tons of more imformation about the game. In September 2005 a Sonic X-treme demo was privately auctioned and sold to an anonymous collector for $2,500.
In 1996 A Sega Genesis version was planned. It was like Sonic 3D Blast. It may have had more of a focus on platforming rather than exploring. From the single screen shown it's impossible to determine if this is entirely true, or if there was to be more open areas to move in.
Sonic Mars was a concept in development for the Sega 32X, which was later reworked into Sonic X-treme. The 32X version was cancelled, probably due to a combination of the add-on's limitations and also its ultimate failure in the marketplace.
Another Sonic was planned called Sonic Saturn.
This game was meant to have a more realistic rendition of Sonic's world than X-treme. It was never actually announced publicly that it was in production, but didn't get very far before being cancelled. What was cool was the bouns rounds were going to be a 3D Sonic Pool.

A Sega CD port of Sonic 2 was planned to be out. There is no screenshot but here's a issue of Sega zone about Sonic 2 with time travel in it up here. It was ment to have new levels and new music that the Sega Genesis/Mega Drive did not had.

Sega Technical Institute was making a game based on the cartoon show that aired on ABC in 1993. After working on Sonic Spinball. The sprites and level art would generally be larger in size than that used by the Sonic games at that point. Even though it appeared to be slower paced than preceding games it was meant to contain faster moving segments. It was also to be more story driven than previous games so the game could be tied into the TV series more. The Memory cart size ment to packed over 16-MEGS(2MB). The moves were ring blast, buzz saw and Spike blast. I wish could be have been finshed.

A few games I like to mention.
Sonic 3 limted edition was ment to be a compilation of Sonic the Hedgehog 3 and Sonic & Knuckles for the Sega Genesis. This would be similar to what happens when locking the two games together with Sonic & Knuckles Lock-On Technology. It would essentially be Sonic 3 & Knuckles but it would have been contained in a single cartridge and thus probably would have had a cheaper price. It is unknown whether it would have had a port allowing for Lock-On Technology so compatibility with other cartridges is also unknown. If that port were not included then playing as Knuckles in Sonic the Hedgehog 2 or playing the extending Sonic 3 special stage called Blue Sphere would not have been possible. Ultimately the game was cancelled.

Sonic DS the 2ed Sonic game on the Nintendo DS while Sonic Rush was in production. The gameplay was using the pen back and forth to make Sonic run. It would have been Sega's killer app on the DS.
Before I end this. Please give credit to Sonic Cult, Lostlevels, and Sonic Retro fo the pictures and research imformation. I forgot to mention Sonic Crackers but it is a Genesis of Knuckles Chaotix. Sonic Eraser is a learning game that was completly forgotin. Not scrapped but Rare! Sonic Jr. for pico which I don't know about. The 32X version of Sonic X-Treme was called Sonic Mars named after the codename for the 32X. Sega Mars. Sega used have a policy to named systems after planets.