My idea would be adding SSMP in Minecraft. This involves transforming a Single Player into Multiplayer when the players that wish to play together have the same map. When they both load it, the game switches to Multiplayer Standard. This is good for private gaming and for quick playing time. When only one person is left he resumes to single player.
Those two worlds are linked, and any other copy it was downloaded from. That means if someone built a house there, it will be there for anyone else.
That kind of excited me, because the thought of this idea, or SP on a local server makes me happy.