This fandom is no stranger to time travel and AU's, of course, but most of the ones I've seen tend to split Lois and Clark apart and/or drop them off in either the future (You have kids!) or some point in the show's run (You can redo this!). I propose something a little different, if you guys are interested:
1. Lois and Clark have been sent back in time together.
Whether they stay together or get separated at some point is up to you, but they're still in the same time if not the same place. So, who sent them, and why? Did they go willingly or unwillingly? Are they a married couple trying to put right what once went wrong, or is Lois hoping Superman will save her from being stuck in the Dark Ages with her hack partner?
2. It's physical time travel
Lois doesn't poof into Lady Loisette's body or anything. Do they run into each other, though? Does one have to impersonate the other? Do Lois and Clark have to answer awkward questions about their clothes, phones, etc.?
3. They travel to a point before the show's run.
No episode revisits, no glimpses of the future, but all of history (and prehistory) is your oyster: Do Lois and Clark meet their younger selves? Does Superman take a bullet meant for Abe Lincoln? Is Clark put on trial for witchcraft? Does Lois have to outrun a velociraptor?
And of course, how do they get back?
