I hope there's a multi-player expansion introduced somewhere down the line, because like NTJedi said it would probably increase sales of the standard game.
Though as for myself I can't imagine ever wanting paying money for the "privilege" of having some obnoxious kid hurl poorly-spelled insults at me. (The AI-controlled races are much
better conversationalists than the average online player, and much more polite, even when they're threatening to eat you skin.
) If a multiplayer expansion comes out, all well and good, but I'd probably give it a miss and just wait for a nice new SP campaign.