The DisplayGroupsInPreviewPane hidden preference has been discussed recently as a possible alternative to return to 3PV.
DisplayGroupsInPreviewPane: DEVONthink will display the contents of selected groups, smart groups, and feeds in the view/edit pane of main windows. The preview only displays files, not groups.
Regardless of whether one prefers 2PV or 3PV, for sure it is desirable to have the Preview Pane used for information rather than blank - so that is a plus.
However, it seems to me that the icons are only helpful if one has data with lots of distinct icons. If instead I have a bunch of documents with no distinct icon, then this hidden preference is not particularly useful.
See below where the top and bottom panes both show the same information - the top is useful, whereas the bottom is only marginally useful.
If we had an option to DisplayGroups in the preview pane in either Icon or List format, then I would not need to open the Group on top in order to see its detailed contents; I could instead browse through multiple groups by just highlighting the group folder on top and seeing the contents below in the Preview Pane - as it used to be with 3PV in DT2.