Hmm. As for something that can be done but I really don't have the time (though I do have the knowledge) to do:
A loooooong while back, Scott Nobblit and I were talking about a scenario that would involve something like a rip in time. Rather than the dinky little Electric Sheep levels in M:I (though the levels following the Electric Sheep were always, always freaking awesome...I love the combination of slick combat, perception twists [the skyline that vanishes when you hit the button on Eat The Path always makes me happy], advanced movement skill, and relatively good ammo supply), you would have various 5-D overlapping rooms where, if you actually enter the room, you see a dim, faded, and overall trashed place, but if you look through a window on a different wall you can see a battle raging inside. I'm sure someone with an overactive imagination and enough guts to make nearly-the-same rooms (I tend to give up when it complains that it can't fill the polygon correctly, heh) could make a fairly nice story that incorporated that somehow.
As for previously created levels that I seriously enjoyed, I like those dream levels...and I liked all the Citadel levels in M2. Maybe it's something to do with the lava; I don't know. ;)
As far as storylines that follow it, Rubicon's Salinger plank is really, really well done. The bit about the teeth chattering? Mmmm. And the extension (at least it seemed to me) on the dream levels that play out in M2 with their ramblings. Mmm again. In Bungie-land, M2 always seemed above and beyond M:I and M1 to me...
And that's all that comes to mind in five minutes. :)
