Well, I've put A LOT of thought into this and I think this could be the full spec. I don't think there is any other enhancement to this feature that could be accounted for. If you've got one, I'll bet it's a good one.

The following ten actions should simply replace the current "Append", "Insert" and "Replace" menu invoked by a long press.

• APPEND - Takes whatever you select from the Playlists menu, keeps it in order, then and adds it to the end of the running songlist.
• APPEND SHUFFLED - Takes whatever you select from the Playlists menu, shuffles it, then adds it to the end of the running songlist.
• INSERT - Takes whatever you select from the Playlists menu, keeps it in order, then inserts it into the running songlist, right after the song this is currently playing.
• INSERT SHUFFLED - Takes whatever you select from the Playlists menu, shuffles it, then inserts into the running songlist right after the song this is currently playing.
• MIX IN - Takes whatever you select from the Playlists menu, keeps it in order and mixes it into the remainder of the running songlist, sequentially.
• MIX IN SHUFFLED - Takes whatever you select from the Playlists menu, shuffles it, then mixes it into the remainder of the running songlist.
• PLAY - Wipes the running songlist (stopping the current song), takes whatever you select from the Playlists menu, keeps it in order, then plays it.
• PLAY SHUFFLED - Wipes the running songlist (stopping the current song), takes whatever you select from the Playlists menu, shuffles it, then plays it.
• QUEUE - Removes everything left in the queue, leaves the current song playing, takes whatever you select from the Playlists menu, keeps it in order, then adds it to the end of the running songlist.
• QUEUE SHUFFLED - Removes everything left in the queue, leaves the current song playing, takes whatever you select from the Playlists menu, shuffles it, then adds it to the end of the running songlist.

** In all cases, wether you are in "shuffle mode" or not, does not matter.

Furthermore, there should be an eleventh option "Set Default Action" that would invoke another menu containing the ten above defined behaviors. By selecting one of those there, the user would define which action is used by the standard "short press" of a song or playlist.

Lastly, there should be an undo, not just an undo that undoes the last thing you did to the running songlist, but an undo that keeps going and going and going so that you could undo the last 30 things (or the last 100 things) done to the songlist. This one might be a stretch, but some sort of undo is important.

I also wish that I had even an iota of the programming skill to make any of this happen.
Tony? Mark? Mike? Mr. Lovejoy? Anyone?