Idea #1: A DS game. You know how in Sonic 3 and Knuckles there were those nice transitions between levels? Well, I was thinking it would be all of the levels in Sonic 1, Sonic 2, Sonic 3, AND Sonic and Knuckles all rolled in one package. There would be those transitions between every level. You would be able to play as Sonic, Tails, Knuckles, or any pair of the three a la Sonic Advance 3 minus the tag action thing. Each would control just like they did in Sonic 3 and Knuckles (meaning all three can do the Spin Dash, Sonic can do the Insta-Shield and can do those special shield jumps, Tails can fly, and Knuckles can glide). I was thinking of doing something with the Special Stages, but I don't know what could possibly be done. The actual game would be played on the top screen, and on the touch screen, you would have your score, time, ring counter, any Chaos/Super Emeralds you have, and maybe some kind of map of the level. Also, I was thinking that you could store shields like Mighty can in Sonic Mega Mix (a hack game that I never played but saw footage of).
Idea #2: SONIC ADVENTURE 2 BATTLE SPOILER AHEAD!!! (well, sorta) A Wii game. You know how if you collect every Emblem in Sonic Adventure 2: Battle, you would be treated to a 3D rendition of Green Hill Zone? Well, I was thinking of all of Sonic 1 being done in a similar way (Not sure how the engine should run). Either that, or all of the four 2D Genesis games.
Addition to first idea: Maybe the touch screen could function in a similar way as in Sonic Rush. I can't make up my mind.