Agreed with GameNerd, but even if it's just a straight-up HD port with no frills and no addons, hell, I'd still play it. It looks fucking gorgeous. And just to make it clear, I'd be fine if there were no zone-to-zone transitions. That was more of a staple of Sonic 3 & Knuckles, and I don't mind it staying like that with the exception of Sonic 4.
Actually, there are plans to include the scrapped zones from the game, namely Hidden Palace Zone, at least. Others we might expect are Wood Zone and Dust Hill Zone, but I don't believe that's set. Genocide Zone became Metropolis Zone, I believe, as "genocide" was probably a description too touchy for young audiences.