I added a tuba part, a music box, and I used the existing backup vocals as the modulator source in a vocoder.
None of this is particularly appropriate to the song, but I was looking for crazy out-of-the-box ideas, and I think it kind of works. Kind of.
I'm at work now, but my computer at home is allegedly uploading stems as I type this. I'm a little concerned that it hasn't finished yet.
I may redo the music box later.