I think that one of the things that is really missing in WP as CMS is a way to connect post types using a many-to-many relation. That one needs to implement a such common thing from scratch (UI, API, storage model...) is crazy. All other major CMSes allow that.

If that would land in WP core, among other things, connect multiple attachments to a post would be much easier. Thew post_parent that core uses to connect images and posts is just broken because you can attach images to many posts but only one can "own" the image as parent, and in a non predictable way: sometimes you attach an image to a post and the post becomes image post_parent, sometimes just not.