Dungeon Crawl: Stone Soup is an open source game (and not open source in the sense of "you can look at our code but don't expect us to value your input", open source in the sense of "make pull requests!") and it's probably the best game I've ever played. I don't think I disagree with you, though: the reason it works so well for Stone Soup is because the game lends itself to personal extension (it's a roguelike, so you're dealing almost exclusively with game logic).