This isn't quite the same, but the original Demon's Souls on PS3 is known for having a substantial area (would've been the 6th major area after the included 5) that has been datamined, with partial collision and some incomplete enemy models/animations resulting in a partially completed resurrection of it:
https://www.youtube.com/watch?v=LjkdKKgmofA(there are of course multiple YTers who have made their careers based on Souls cut content)
There was a partially finished dungeon called Singing Mountain on Chrono Trigger that Square confirmed was cut from prehistoric age, but the music for it and some layouts/graphics can be found in the ROM.
Tomba's Village of Civilization is just one piece of a large amount of unused Tomba stuff that is actively being researched:
https://www.youtube.com/watch?v=Nq5rTECKb_w 
Sonic 2 also has a couple zones found on the ROM that weren't used iirc, not sure how far along. Those are a handful I can recall.