Since, there seems to be much discussion about the Classic Press plugin directory here is my two cents. I think a very important feature that is missing is integration of the Classic Press repository with the “Add Plugins” admin page. This will allow the user to find Classic Press plugins using the familiar “Add Plugins” admin page. I think it is asking a lot of ordinary (i.e, non evangelical) users to look through two different repositories using two different user interfaces. This would require modifying class-wp-plugin-install-list-table.php and plugin-install.php and implementing a server to support the same protocol as “https://api.wordpress.org/plugins”. This protocol is quite massive (a print_r() of the initial response displayed 23,700+ lines) but it seems only a small fraction of the returned response is actually used so maybe only a small subset is required. I wonder if the implementation of the server “https://api.wordpress.org/plugins” is also GPL. Seems this would require a significant effort but I think the payoff would also be significant.
This is something which has been discussed previously with the web based directory being phase 1.
Agreed it is a lot to ask them to look through both, however as @azurecurve mentioned we choose this as phase 1 as it allows us to build out a directory of supported plugins, get feedback early, and iterate before we touch api endpoints, and core code. It is on our roadmap, and we are planning for it 
The WP API is not open source.
But yesterday, a commit was made to WP trunk for changes to add support for a new plugin header called Update URI. This change should be made in CP also, so each plugin can specify where to look for an update. That means it could be defaulted in CP to wherever the CP directory would get its information.
The WP meta team has already added some code to support this, but I’m not sure if it’s committed yet.