Maybe I am really slow and everyone here may know about this project, but I have just discovered Sonic 1 Megamix...and my jaw is still on the floor.
http://info.sonicretro.org/Sonic_1_Megamix
Small description from the site -
"Sonic 1 Megamix is a multiperson assembly modification of the original Sonic the Hedgehog videogame for the Sega Mega Drive, headed up by Team Megamix and released for the Sega Mega CD. The game features the ability to play as one of five different characters: Sonic the Hedgehog, Mighty the Armadillo, Shadow the Hedgehog, Miles "Tails" Prower, or Knuckles the Echidna. It features an extensive reworking of the original game with new art, music, abilities and layouts, among other things."
I have played the first 3 zones and OH MY GOD!..this is beyond amazing. Its every bit as good as calssic sonic games. Sega themselves couldnt make a 2D game of this quality today if they tried. The game has multiple playable characters, all with uniqe abilities, music is classic sonic lifted from other games, gameplay is enhanced, no bugs or glitches, bonus levels are reworked, legth of the new reworked levels has been boosted atleast 3X, difficulty has been boosted a little...check it out if you havent already.
You can download the rom on the bottom of that page. Go for Version 3.0 if you can't run sega CD games on your emulator, as its a .bin file (genesis) and can be played with any Genesis emulator. (on PC go for Gens if you are gonna get one as its the best). All versions have different music and level skins too...I tried them all and V3 seems best to me.
Here is a youtube video of the game in action...
http://www.youtube.com/watch?v=uLE5XFkbN6U
