Honestly, I'd love if they develop a full-retail "New Sonic the Hedgehog" game. Like for the Wii U would be cool.
* Featuring Classic Designs, no dialogue (but perhaps let the characters make some kind of noises), and the main "plot" told through voiceless cut-scenes like in the old Sonic games.
* Play as Sonic, Tails, and Knuckles, and they all play through the same courses but have different routes accessible like in Sonic 3. Sonic has the "spinslash" thing from Sonic 3 and is the fastest, Tails can fly and swim, and Knuckles can glide and climb walls. Power-ups return, the Fire, Bubble, and Electric, maybe some new ones.
* 16 Zones, 2 Acts each. First Act has a Mini-Boss (a giant Badnik), Second Act has a Boss. There's the Zone-to-Zone/Act-to-Act Segue's like in Sonic 3.
* 8 Chaos Emeralds, 1 in every odd-numbered Zone. And 5 Red Rings hidden in each Act that unlocks bonus content, like the Star Coins in Mario and the Puzzle Pieces in Donkey Kong. Some Red Rings require using the other characters, so there's replay value.
* Plot is simply Eggman being Eggman, but halfway through the game Metal Sonic takes over, and unleashes various "Metal Sonic" prototypes in Eggman's Lab. The main bosses in the last half would be the Metal Sonic prototypes with unique abilities.
* Multiplayer, with Co-Op and Versus, utilizing the Wii U Tablet for Player 2. In Co-Op, the second player can be one of the other characters, and plays through the level alongside Player 1...using the Wii U Tablet, he doesn't have to worry about getting shoved off-screen, and there's a "help" button to warp him back to Player 1. There'd be special team-up moves too, like Knuckles could carry the other player when he climbs, and Tails could lift them up. Sonic, I guess, could perhaps use his spin-dash to slingshot them. In Versus, the players play the same stage, but their own "version" of each stage...Player 2 uses the Wii U tablet to do this. The players never interact, but instead race to the finish, and can see each other as a "ghost".
* Using the Miiverse, players can drop hints at Checkpoint Signposts for other players, and Times for Acts can be shared online. Players can then race those times on their own, like in a racing game, against "ghosts".