I like your idea about an admin_basepath that defaults to the first route_basepath, I think it makes it more explicit what is happening and we can enhance the selection widget going forward to handle the multi basepath situation or just allow users to do that themselves.

The route prefix listener for multisite we have needs refactoring but I am waiting until the candidates stuff you wrote lands. Once it does, we will be able to just set a host requirement dynamically on the route in a post load doctrine odm event which will alleviate issues with URLs being generated absolutely when they are not part of the current host basepath. The other issue is the one of transforming a url like /about which has no prefix into a cmf path which would involve a listener somewhere that looks up basepath by the host and fixes the path. This part im not sure how/where we will implement. Its not really a discussion that is part of this per se but i figured id mention it.