Sonic 4 isn't the only Sonic game on the horizon to look forward to. I thought I'd quickly run down some of the promising Sonic fan games/hacks that I'm personally anticipating. There's no guarantee they'll all ever be finished, but they're exciting nonetheless.
Sonic Boom by SÆGA
This didn't win a hacking contest for nothing! Not only is it technically impressive and face-rockingly awesome, it changes up the Sonic gameplay so much its hard to believe it's a mere ROM hack.
Sonic 2 LD by Doc Eggfan and Glitch
How can you not love this idea? The new sprites (by MoDule and tokumaru) are amazing; why Sega never took advantage of their 8-bit hardware like these guys can, I'll never know.
The S Factor: Sonia and Silver by Aquaslash
Another character hack, with fun and interesting Zone themes and good layouts. I love the music to the Zone in the video. :)
Sonic 1: Brother Trouble by MarkeyJester
Looks gorgeous, and is full of technical wizardry. Besides, I love Manic. XP
Amy in Sonic 1 and Sally in Sonic 1 by E-122-Psi
Absolutely adorable character sprites, and the promise of fully featured new movesets make these two something to keep your eye on.
Sonic 1 Alt by Ell678
Don't be fooled. This is far more than your ordinary layout hack. I don't want to oversell, but the new level layouts are outstanding. I seldom get transported back to my childhood days of playing Sonic on the Genesis for the first time, but this hack can do it.
Sonic Fan Remix by pelikan13
I think any image of this gorgeous project speaks for itself.
Well, there you have it. Sorry to all those whose hacks I didn't include - this is by no means a complete list! I'll probably run down another list in the future.