2 suggestions

[visual quail]

Being able to set wall sliding speed and being able to make bgms not stop each other
You could set the wall sliding speed to negative to make you character climb walls without having to put ladders in all of them
And bgms playing over each other could be used to make a song play different versions of itself depending on where you're at without restarting and also could be used for making longer sound effects

[delicate kindle]

Yeah having different bgm playing at the same time will give a really good combo for dynamic music, top suggestion

[torn olive]

OH YES.

[fringe shoal]

multiple BGMs would be awesome!!