I know you’re already working on Marketplace integration directly from the Sitefinity back-end but in case you’re looking fors ome further inspiration, if you haven’t done so already, please check out the Orchard CMS on Codeplex. (http://orchard.codeplex.com/)
Don’t worry it’s not anywhere near Sitefinity but it’s got (from a user standpoint) seamless integration between local assets and the online gallery.
This integration in turn is base don the Orchard Gallery Project (http://orchardgallery.codeplex.com/) which also powers the NuGet gallery (probably something the developers will have heard of.
So why is this so perfect for Sitefinity?
It based on an OData feed as the gallery listing, and allows for multiple sources which would be super so we could set up ‘internal marketplace’ for custom (in-house) build modules we don’t share on the public Marketplace.
The versioning part would also be great since it would allow for easy updates and bug fixes (both internally as well as on the general marketplace.
Perhaps it would be possible even to have like 4 feeds, 2 focused on FE (themes, widgets) and 3 BE (modules, themes & extensions).