Character/Map Creation API?

Is the developer API only for retrieving statistical data? Or is it possible for some serious in-game modding? Sure, you'd have to have some alternative option for linking players through LAN. Can't be running modded clients on the main servers. But there are a few map/mode ideas I'd like to flush out more concretely. I had a ton of fun modding Minecraft a few years ago, and feel like it's time to dive into another game source. But if it's not available... that would suck. :(