So I have been thinking for a while ever since Little BIg Planet 2 with the feature of "level portals"
So basically it is just that when you are playing through a user created level, if the creator runs out of money or room or just wants to change maps there is an object you use that takes you to a small loading screen while staying in the game. You are then taken to that level without ever going to the menu or leaving the game.
What if Halo could do that. I mean one night I thought of an infection level. OK so everything is going great everyone is in a city working when Zombies come out of the sewer. The first part of the game you are just trying to escape from the city to prepare to fight. Once you get out of the city there will be a portal that takes you to the exact same spot on a different level. You walk back and the city is still there. Except it is all ruined and destroyed.
This would be awesome for puzzle games but not really for slayer or competitive games.