An emulator for the XBox 360. Here's the catch... it could be made available to XBox Live users that have had their consoles killed by MS's shoddy construction. The emulator would ONLY work with ORIGINAL game discs.
If for the PC platform, its unlikely it would really be viable. The issue console manufacturers find with emulators isnt legality or piracy, its the effect on sales of the *hardwares* in particular (game prices take the risk of non-popularity in consideration too, and console prices the same).
It all comes down to marketing-specific strategies (thats just other than how xbox360 wouldnt really be viably realizable on PCs. Their games are already pretty much native PC games, and many PC versions already exist, so even less reason why need of emulation rather than direct translation on a per-game basis).
Problem is though, that hackers would have a field day removing the software restraints on the media and the license for what machine it would be used on, then it would become common knowledge on how to effectively play 360 using official software for PC's, the ease for individuals to pirate the software and games, it would reduce the sales for the 360, thus removing any profit margin for Microsoft, and it would become much harder to justify selling the console if people can easily play the games for nix on a PC.
Although, this could become a reality for Nintendo, with the increasing popularity and compatibility for the Dolphin emulator.