I thought it would be nice to be able to do more with filters than just cover attributes. My idea was to have filters be able to run queries. Any query language will do but I think the simplest, most consistent implementation would be to use Semantic's <ask> queries - although they are very limited. The filter's definition would say: show all pages that meet this query's criteria for value X. Those values could either come from a category or possibly a list. On the simplest level, it would enable using ANDs and ORs for filtering. Perhaps it can be implemented in a way that would allow for nested queries too.