S1 on GBA with Spindash? Sounds like something /me would love, as the original was and is, despite me sucking at it like all 2D games I play, one of my favorite games.
If there were any other general improvements, I would be more than excited. Somehow, I forsee some other random improvement coming that won't be released for now, because it would probably need some other thing to get Sega some money, considering that, despite how popular the original Sonic was, GBA is already a dieing system to buy for/system to use for making games (regardless of DS marching in, because not many people would look in the GBA section for a "high-standard game") and the majority of Sonic fans with access to the internet probably were told of a ROM of S1 with Spindash, meaning that this would not swiftly attract much attention to the big Sonic gamers. (Also consider the fact that any emulator can use save states.)
Then again, portable Sonic 1 would always be awesome, but still, the points above seem to linger around in my head.