i pitched an idea like this once... you could build portals from one server to another.
basically, you'd build a portal out of some new block type, when you light it, it turns on and its red. you jump in and after it does the whole wavy screen thing, it takes you to a screen where you can type in an ip. if you type in a valid ip, the portal turns green (to let others know it works) and you appear in the same place on the other server. it would also clear a few blocks around the portal on the other side so you could step out, but who knows what else would be around you!
