A good idea in theory, but a 'seriously competitive' game like Halo wouldn't work like that. Hang 'Em High just isn't balanced for jetpacks and dual-wielding. They have been going back and tweaking old popular stages to fit a little better with the new gameplay enhancements, but to do *everything* might be a little much, resource-wise. They could just throw them in there for fun, and have people recognize "these are gifts, we didn't spend a lot of time on it" like Gran Turismo 5's legacy cars and tracks.

In fact, GT5 is a great example. I think they have every car and every track that's been in a GT game. However, the cars and tracks from the earlier games are not rendered with the same 110% polish that the brand new ones are. This is something more feasible for a racing game where the layouts are less complex than a shooting game would be (i.e. you don't have to worry about three dimensional line-of-sight and weapon locations, not saying anything about the quality). So there's one for you.

In any case, Halo kind of has a way out since it has Forge, and their crazy userbase can just recreate their old favourites. Like Hang 'Em High.