Sonic and Knuckles 3D, isn't culled from any upcoming licensed game. In fact, it's not even from a fan-made game -- rather, it's just a pre-rendered video from a skilled 3D artist and YouTube user by the username of BlobVanDam.
Spin-dash past the jump if you want to see the Mushroom Hill Zone Act 2 that never-will-be. Actually, that's a pretty defeatist attitude to adopt. We're sure that someday, in the fullness of time, this delightful vision will be realized. Until then: Sonic 4!