As you can add custom sheetset properties and custom sheet properties, it would be nice to have the possibility to add custom subset properties.
Often standard and custom sheetset properties and sheet properties are used to populate the titleblock of your drawing using Fields.
With subset properties (standard and custom) it would be possible to expand the potential of the subsets.
With subsets dividing your sheetset in fases, zones, themes or whatever division is applicable for your project, subsetproperties (standard and custom) could reflect that in your titleblocks.
This would add a whole new level of flexibility to your sheet management.