I don't know about you, but if I had the chance to play a Borderlands game that looked like this conceptual piece by Olsonmabob I'd do it in a heartbeat. The art style is fantastic.
Sure, it wouldn't be an FPS anymore, but that may just make it more interesting! Like a Metal Slug or Contra sort of deal. I dig it, especially since it would remain a 4-player co-op game from the looks of it.