Speaking of the Legend of Zelda, Zelda Classic is a Windows/DOS-based replica of the original NES Zelda game with which you can build your own levels and customize existing ones. It's like Excitebike + Zelda. Hopefully these folks won't get sued by Nintendo (although I suspect they will).
On a related note, I suspect that there's a significant market out there for old games that have been augmented. MAME has a huge following and several companies have released older arcade favorites for the newer gaming systems. EA is even doing some new things with the original version of SimCity, allowing people to play online while chatting with other city planners.
With Zelda as an example, Nintendo, borrowing from the feature sets of more recent games, could create a new version of the old game that would work online, allow people to create & trade custom maps, let more than one person play in a world at a time (collaborative puzzle solving), and allow players to change the difficulty of game play (as well as a bunch of other factors). It wouldn't work for every old game, but I'd wager there are a few games that people would play and pay for.