Minecraft: Invite People to your Singleplayer.
I came to this Idea when I wanted to play with a Friend on my existing Single Player save without the hassle of setting up a server. I imagine it like that, you can play your Singleplayer save, and when a friend comes around, or you need someone to join you, you could click a button in the ESC menu that says something like [Open Game for Friends]. This way you would start hosting your world, and people could join you if they know your Minecraft username by entering it in the Multiplayer Server field. If you like this idea you could either code it with your own ways, or look into getting minecraft going with steam and steamcloud.

Greetz!