Awesome feedback, thanks Lee. I think we could change the data import to allow overriding values via the JSON file, so anything valid in the Nuget config would also be valid in the JSON (and prioritised in the latter). I'll run it past Andy, given he's built all the service layer.
Big benefit here too is the JSON can be updated independently of the actual package, so less effort for devs.
.umbraco folder is nice idea. Would be a clean way to co-locate images, icons etc too.
Can't recall if the marketplace repo is public. If not, I'll transfer this over to an issue or two