There are a lot of things DICE could do with DLC for this game, but I don't think they'll take the Civil War, World War I, or World War II paths. The Civil War would be difficult to make enjoyable granted the way battle was fought back in those days, and would be tricky to implement correctly. Adding any past war to the game would seem somewhat silly as a DLC, because it would provide less weapons, vehicles, etc., granted they weren't present during those times. It would seem more like a downgrade than an upgrade.
Whatever DICE decides to do, they're going to have to do a hell of a good job with it granted the current landscape of the FPS genre. Present time shooters have been done, we've seen those with Medal of Honor. Future shooters have been done, also, with the likes of the Modern Warfare titles. The only path that hasn't recently been trekked on would be very old wars [i.e. the Civil War (which I don't think they'll do for reasons above)] and future warfare. Future warfare seems to be more so the direction of Call of Duty, though, not Battlefield. Over the years the Call of Duty franchise has strayed from their realism factor, and has focused more on creating a really fast-paced, crazy experience, while Battlefield has stuck to its roots in trying to be realistic.
Really not sure what DICE will do. All the options available to them don't really seem feasible. They've certainly got a challenge ahead of them.