I would assume that split screen multiplayer will not be added in the future. I imagine it's enough of a pain to get the game to render everything twice for local split screen, much less network that. Add in that there are better ways to burn money than figuring out how to do that for an infinitesimal portion of your user base and yeah, don't hold your breath.
Also, every mode in the game can be played in split screen, just to clarify for some people in this thread. You cannot play split screen online, but that isn't necessarily what a "mode" is in the context of the statement. You can play the BYB, the Garden/Graveyard Ops, and the Multiplayer Modes split screen - the last two being done in private matches, instead of networked ones. I would also hesitate to put the story quests down as a "mode" - those are activities, not discrete modes of play.