This is something that I've wanted for a long time, which also seems like not a big deal to add (though usual caveats that I don't know WTF I'm talking about there apply), and so I figured I'd ask:
It would be handy to have certain individual Panes (in a Panes view, of course) which do not filter the list based on the active selection in the other panes in the View. In the Edit Category dialog, you'd add an Ignore Other Category Filters checkbox (or some such thing). When enabled, that Pane would always show all available items in that category based on the View's Files to Show filter only (just like how it is if the selection is reset).
The reason I'd like this is for tagging, particularly for the Genre tag (but also Keywords and stuff like that). It would be useful for any field for which you have a "set" of possible values to choose from in whatever personal metadata "system" you've devised. So, for example, say I want to tag all ABBA songs as Pop\Disco. I get a new ABBA album, as you do. In my "New Music" View, if I have the View un-filtered (with no selections in the Panes) I can select the ABBA songs and tag them with the Pop\Disco item in the Genre Pane. But, if I've filtered the view with one of the other Panes, such that no visible files are already tagged with Pop\Disco, then that item vanishes from my Genre Pane and I can't use the little checkboxes to tag the files that way anymore (and I end up typing them in by hand like an animal). This happens to me a lot when I'm tagging because it is often convenient to sub-filter the view to show only the files for which you want to "universally" apply a particular tag (so you can just apply the tag to all of the files in the current view). For example, I'll narrow down the selection with a File Location pane to show only the new ABBA album I just imported, then I want to apply a handful of things "globally" to all the files (fixing Abba to ABBA, applying Pop\Disco Genre, and tweaking the album title to remove the "(Digitally Remastered Loudness War Version)" suffix, etc). But as soon as I subfilter to show only the files where I want to apply the tags, now my Genre category is empty.
You can battle this somewhat by carefully ordering your Panes, but there are some circumstances where it would be nice if a particular Pane didn't filter itself, and always showed whatever it would show based on the the results of the View's Set rules for file display search, regardless of its order in the set of Panes. It would mean if you use the Pane to actually make selections, and you select an item for which there are no results, you could end up with an empty file list, but that's no big deal. And the little checkboxes would clue you in to this anyway when using the Pane for selection purposes.