*whistles* I didn't want to do this so soon... but-
http://www.projectzeldaonline.com/
Online Zelda Multipler MOD for Halo: Custom Edition, for the PC. Z-targetting, horses, hookshots, and you name it.
You can ussually catch a bunch of updates on the Gearbox forums
http://gbxforums.gearboxsoftware.com/forumdisplay.php?f=10
That's not to say other games aren't an option. Mario, Metroid... HA! Which reminds me, there's virtually a whole Nintendo revival community over there. There're projects for Metroid and Star Fox for Halo CE as well.
All that aside, the biggest factor for modding that'll kill any project is programming. Unless you have a handful of decent and dedicated programmers, it doesn't matter what other talents you have in your crew, the project will fall flat on it's face. Of course, if there happens to be some coders around here, it may be a worthwhile venture.
None of this has much to do with emulation, though, unless you had a fairly specific idea in mind.