Took a first step to this in 2.2, where you can use Shortcuts to specify complex filters on albums (not yet playlists) and then view those in Longplay like an ad-hoc collection.

The just released 2.2.1 update includes the "Smart Collection" examples Shortcut which demonstrates this. They can be found in the in-app Settings under Siri & Shortcuts.

This should go well together with adding a "Search" feature. Will look into that in the near term.