Adding multiplayer is likely impractical, as the engine has almost certainly been streamlined for single-player. Bethesda has also said that they prefer to focus all of their efforts on single-player and would rather not split their focus by adding multiplayer (which would requite a substantial chunk of developement resources) so we probably won't see a title in the core series with multiplayer either.
A spin-off isn't out of the question. In fact both The Elder Scrolls Legends: Battlespire and The Elder Scrolls Travels: Shadowkey have multiplayer.
This is a topic that comes up frequently so we have given it a http://www.gamesas.com/topic/1320106-official-tes-multiplayerco-op-thread/. You're welcome to jump into that discussion, but please make sure you reach the ground rules in the opening post. Multiplayer is a frequently contentious issue here.