Ability to collapse on a field. For example, I want the most relevant result from all different report types. Or similarly, the most recent result of each report type. Or maybe, I want to de-dup on headline.

So, the sort order would dictate which one from the group is returned. Similar to what is discussed here:
http://blog.jteam.nl/2009/10/20/result-grouping-field-collapsing-with-solr/

From my understanding, it seems that in order for field collapsing to be efficient, the result set must be relatively small.

This is also referred to as "Combine" on some other search products.