* Publish/Subscribe Eventing System: It would be nice to be able to "subscribe" to certain events, eg: "user.created" or "module.installed" or whatever.

That's a good idea! I take notes :)

* Plugin Marketplace: The ability to buy/sell/give-away-for-free plugins to help manage/install things with this tool would be really nice. You could start with the ability to install packages with yum/apt-get/emerge and go from there.

Yup, this one is already on the map :) Actually it's even somewhat a core part of the whole project (which is bigger than an administrative interface)