but i think plone.app.mosaic is the better choice than collective.cover. I have forked webcouturier.dropdownmenu and work on a P5 Version. But i'm not that experienced with the test-machinery, so it takes a time.
What we need is Link to extern/intern Pages in the Leadimage Behavior of plone.app.contenttypes, but this is probably not the right Topic here.
@vangheem and I have been working on FSD to make it work with P5, but we are taking a very minimalist approach (and as part of that, we have a version of archetypes.referencebrowserwidget working with P5 as well). Will take a bit more testing. https://github.com/plone/archetypes.referencebrowserwidget includes the latest fixes already.