Thanks! Meteor really proved itself as a capable platform. We hit a few issues, but those were resolved rather quickly.

Yes, we already allow people to write their own extensions. For quick stuff, there's a built-in code editor: http://joxi.ru/SFJNUxjKTJBvQrogwYE. And for large extensions, there's an "Add your extension" button in the Extensions store.

In fact, we'll allow people to monetize their extensions, in a micro-App-store fashion.