I sort of like the time machine idea, but not as described above. Perhaps a quest where you are in 1403 via time machine, but you must try to find clues and help to get back without being discovered. Perhaps some disastrous consequences for not concealing who you really are. Basically, take a modern character, and make him play true to the game's atmosphere. No "magic"... no tech... no slang, etc.
Maybe it's just me, but that sounds like a really cool way to mod the dialog system, stealth system (especially until you get period accurate clothes on), crime system, and more.