The Marketplace is where Kiln will grow beyond the editor: drop-in extras added from the Kiln menu, committed to your repo, nothing locked in. It isn’t ready yet. Comments, in-editor site search, and a block library already ship in Kiln itself as of v0.4.0 — those are not Marketplace plugins.
In developmentKiln is open source (GNU AGPL-3.0), and the plugin surface is being designed in the open. The REST API and kiln-mcp shipped in v0.4.0. 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.