I'm not sure if this is what you had in mind, but the kind of extension I'd like to see is one that makes development of games of certain genres easier. For example, the RPG extension would allow the user to create elemental attacks, status ailments, stat growth, etc. from the UI, rather than trying to manage disparate behaviors. (The number of behaviors in the game I'm working on is already exploding, and I haven't made it very far yet.)