Add the ability to use Solr's Grouping / Collapse capability to selectively collapse only certain results that match some specific criteria. For example, only collapse documents of matching some type of query ("group.filter=type:image"), leaving all other documents ungrouped. Should be able to specify multiple parameters to get more than one group (e.g. group images together and news articles together, but other documents are ungrouped).
It has been suggested using filters for this. For more details, see the mailing list post on nabble below: