Hey Steam, wanna sell twice as many games as you are now?
With things like the Humble Bundles and Steam Sales, collecting games has almost become as much fun as actually playing them. If steam could play off of this idea and give people a bit of an incentive to collect games, there could be a gold mine waiting.
A good idea to this could be to either A) Assign a trading card to each game or B) recieve a random gard for each game you own/purchase. Then create some sort of game in steam that uses these trading cards to battle similar to pokemon or magic or something to that effect. Allow users to trade/sell these cards. People would buy up games they don't own simply to receive more cards in their deck.