I’ve enjoyed coding extensions on request – Aaron Reed’s Mentioned in Room Description; Michael Bacon’s iPoetry; David Fisher’s Default Messages & Custom Library Messages. These are fairly clear-cut and bite-size problems, taking no more than two or three weeks to go from conception to finished product. But an actual game is a pretty big commitment. The Inform 7 Suggestions site floats the idea of an extension site that could take requests, reviews, etc. of same. If that gets off the ground it could be a tool for collaboration.