The Marketplace is where Kiln will grow beyond editing: drop-in capabilities added to a site from the Kiln menu, committed to your repo, nothing locked in. It isn't quite ready yet but here's the kind of thing we're thinking of.
In developmentKiln is open source (GNU AGPL-3.0), and the plugin surface is being designed in the open. If one of the ideas above is something you need — or you want to build and list one — we'd like to hear from you while the API is still taking shape.