I would love to have an Auto Playlists feature which can be used to create a playlist based on search criteria, song rating, play count, count since last playback or folder name. The Auto Playlist would be created using a text editor and have an ".apl" extension.
I'd be willing to work on this if there was enough interest.
It could be implemented as a plugin or as part of the RockBox base - I'm not sure which would be more applicable.
Here's an example of a custom auto playlist selection
Select: File, \music\good\
Select: File, \music\incoming\
Recursive: Yes
Max Track Count: 100
Weighted by: Song Rating, 100, 80, 60, 40, 20, 0, 0, 60
Filter: Min Count since last play, 100
Sort: Shuffle
When this playlist is selected a random selection is made of 100 tracks from the folders \music\good\ and \music\incoming\ which have not been played during the last 100 song plays. The song rating is used to randomly skip over songs - 100% of the songs rated 9-10 are included, approximately 80% of the songs rated 8-9 (20% chance of skipping each song which is rated 8-9), approximately 60% of the songs rated 6-7, and so on.
Here's another example:
Select: Database
Filter: Genre, Ambient
Filter: Genre, Jazz
Filter: Min Rating, 6
Sort: Tag, Title
When this playlist is selected, all the tracks that match the search criteria are selected and sorted by Title.
Another example:
Select: Playlist, \playlists\worksongs.m3u
Select: Playlist, \playlists\playsongs.m3u
Duplicates: Remove
Sort: Default
This would play all the songs in the worksongs.m3u playlist and the playsongs.m3u playlist. Duplicate songs would be removed. The sort order would be depend on the current Shuffle configuration.