The count of a collection-valued (structural or navigation) property can already be used in common expressions, e.g. in $orderby.

It would be useful to also allow the /$count segment followed by a nested filter or search option, e.g.

GET entitySet?$filter=navigationProperty/$count($filter=YYY) gt 5&$expand=navigationProperty/$count($filter=XXX)&$orderby=navigationProperty/$count($filter=XXX) desc

to get the entities with the highest number of related entities matching filter first.

This was proposed on the Olingo mailing list: http://mail-archives.apache.org/mod_mbox/olingo-user/201512.mbox/%3C5662B48C.7020305%40gmail.com%3E