I wondered what would happen if you applied this philosophy to a game engine. Imagine Unity, but with the ability to open a REPL in-game and run all of your code interactively.

I managed to accomplish this somewhat with Lua. Designing the engine so that all the cool things like creating and equipping items, generating maps, etc. are in public APIs you can use interactively has given me a lot of power. I'm not sure if the benefits offset the maintainability tradeoff, but I'm somehow continually excited to sit down and mess around with it. Taken as a single-user experience, being the only person to understand the system and write more code, it is almost a dream come true.