Super Mario Bros. is a classic game. It has held up amazingly well over the years, and is still as fun today as it was in the 80s. Unfortunately, I've played the first few levels of the game so many times that I wouldn't mind if I never see them again. The serial nature of the game made sense in an 80s arcade mindset, but today, most people playing Super Mario Bros. probably aren't out to finish the game, and likely have already finished the game in the past, instead just wanting to play for a little while before moving on to other things.
My idea, then, is to hack the SMB1 cart to make it better suited to the casual player. I take inspiration from, of all things, Quake 3: Arena. First, add a level select screen. I'm sick of endlessly replaying World 1. Second, some stat keeping to encourage replay. It should at least record the time to completion and score accumulated during the level. It might be worthwhile to give the player only one life to use during a run through a level, to make things more interesting, and stop them from gaming the scoring system. Naturally, add save ram and a battery so this data is preserved. Finally, more levels. You could integrate the whole of SMB2j, and possibly fan-made levels from the internet.
On the technical side, move it over to an MMC1/3/6 board with a battery, and you'll have plenty of space to cram in various hacked versions of the games. Write the main menu, make some minor hacks to the games to integrate them with the scoreboard, and you're set. Shouldn't be too difficult, although it isn't something I want to spend time on right now. Perhaps something like this already exists, among the hundreds of SMB romhacks that no one has time to dig through.