Being able to make your skin/avatar/character one of the characters from one of the games you played. You could have a militia in Home, and it could be made out of Helghast and those Haze soldiers, every one of which could be some guy who played the games, backed up by an assload of Nariko variations and Ratchets and Clanks, and you could have Solid Snake come visit Liquid's crib. Actually, screw Liquid....he could go to Meryl's crib.
EDIT---Also, what would be hilarious is watching said militia go up against another group of players, and since they can't actually hit each other, it would turn into a massive text-and-mic-based Yo Mama fight. Between like 500 people.








