Just a thought:

When I use the "folder" tab from filter pane I have to expand quite many folders before I get to the "important" ones. Example:

Music is in: /run/media/user/ExternalStorage/Music and /home/user/Music

so I have to expand 5 (3) folders before I get to the first folder containing music! What about hiding all folders before the ones containing actual files?

Regards, staubi

Sorry for not replying, I wasn't sure if I'd do it now or not.

Currently it automatically opens the folders until it reaches a fork, but of course it doesn't work in your case where the fork appear very early.
As a workaround, you could use a symlink to make /run/media/user/ExternalStorage/Music appear as  /home/user/Music-external. Which also has the advantage of making it easy to change the this if your media was mounted elsewhere for some reason.

Though it's still something that could be improved, also the current folder view wastes a bit of space on the left, as typically the songs are deep into the tree.

So, I've implemented a solution by fusing a folder with its sub folder if it has no songs and only one sub-folder with songs (that might be in sub-sub-folders).
Though I don't like it much as, if you have a tree where albums are sub-folders of artists, it makes artists with only one albums stand out :

So I added a variation that only does this with the root folders, in your case it would do :

+ run/media/user/ExternalStorage/Music
+ home/user/Music

In my case it does :
+ artist1
+ artist2

But I'm not sure this is the correct rule for what I want, in case you had music on two /mnt drives it wouldn't work right as it would do :
+ mnt
   + drive1
      + Music
   + drive2
       + Music
+ home/user/Music

Also, I'm not sure about the option name, currently I think I'll call it "Simplify folders" with three possible values : never, "only with root folders", and always. A more descriptive name would be "Fuse folders with no songs and only one folder" but it's bit too long and not even very clear.

Any comments / suggestions ?

I've found a better alternate rule: only fuse folders if their siblings can be fused too, it was a bit harder to implement but it works as I want.
Now I just have to find a good name for the option ... :(