One possible obstacle is core implementation may require some changes because ClassicPress Importer plugin will be listed in our directory. The "Install Now" button for this plugin might need to be changed to install the plugin from our directory instead of WordPress repository.
Looks like we're missing an API: https://api.wordpress.org/core/importers/1.1/
This is a list of all the importer plugins. Since we're forking WordPress Importer plugin, we should have our own API and a list of plugins. This is where it's used in the core: https://github.com/ClassicPress/ClassicPress/blob/22675efaf13b457f43f961809ca4ef202bf02dd7/src/wp-admin/includes/import.php#L145
This seems to be a simple JSON list.
One possible obstacle is core implementation may require some changes because ClassicPress Importer plugin will be listed in our directory. The "Install Now" button for this plugin might need to be changed to install the plugin from our directory instead of WordPress repository.