DC Heroes from Mayfair games has a mechanic for getting XP (Hero Points) through subplots that translates pretty well to any XP system.
"A subplot tells a story which is secondary to the adventure, but important to the Players' Characters."
"Subplots give Players a chance to deal with other aspects of their Characters, apart from bashing villains..."
"Subplots can be light-hearted or serious, short stories or continuing sagas."
"Subplots allow the story lines in a campaign to be developed between the GM and the Players in a way which is unusual in role-playing games."
"Subplots get Players to speculate about their Characters' personalities and to initiate new story situations, rather than just react to situations the GM poses for them."
(DC Heroes Role Playing Game, Third Edition)
Either the player or GM can suggest a subplot and if the GM agrees with the players idea (as is or with some negotiation) or the player likes the GM's idea (players and GMs both have the right to "pull the plug" on a subplot at any time if it ceases to be fun) the subplot would get greenlighted and either woven into the story or run outside the framework of the story arc. Anyone who participates in a subplot with the character it affects gets the same XP (Hero Point) reward, so some subplots can involve everyone or a few players (nice for when you're nearing a critical point in your main story arc and a couple people can't make a session).
It's a nine-page chapter in the DC Heroes book (and might be in the super hero book that was published after DC Heroes that uses the same system - Blood of Heroes).
