There was an announcement a couple weeks ago about a new Terraria game called Terraria: Otherworld. The developers of this game say that it isn't a sequel to Terraria. It's basically a story mode version of hte game (I guess) where you have to save a planet in another dimension from a malevolent force.
Thats awesome, Terraria is fun, that will make an interesting way to play it I hope they keep it multi-player.
I have no doubt. Half the fun of a lot of games (including minecraft and terraria) is in multiplayer, they would be stupid not to include it.