While really useful as it is, I believe it could really improve with a few changes:

1. Search posts
   I've noticed theres a field dedicated to that functionality, but something in the lines of what ACF does would be really awesome. When you have a lot of posts, the current presentation won't scale well, hence the need for some sort of search functionality.
2. Limit Attachments
   Would be possible to limit ao many attachments the user can add?
3. Attaching Terms
   It would be very useful if we could attach other types of contents, such as categories, custom post-types, etc.

Thank you CMB2 and this useful extension.