It's hard to name a good Sonic title once he graduated to the 3D console.  Whether Sonic was meant for the 3D world or not, there's nothing really stopping Sega from sending Sonic back to his 2D side scrolling roots.  Nintendo made it work with New Super Mario Bros., you just need a little creativity.  I'm certain there are plenty of gamers who'd pick up a retro Sonic game if Sega put a unique spin and a ton of polish into his rebirth.  The video above shows the difference a little visual polish can go.  It's the same feel (mainly because its updated visuals from Sonic & Knuckles) but if Sega brings back the coop play with Tales, scales everything down, and stays true to the franchise's speed, everything might work out better than expected.  Unfortunately, we'll never know until they give it a try.