Well everybody seems to have their own idea of what a remix is.
The song is a good song on it's own, you could do the banjo mix where the only real difference is that it has a banjo track. that's a new mix.
I'm not sure what they want or expect, I think it's anything goes, but I expect they want it recognized as the same song. IOW, not a new song with samples from their song placed in. and not a mashup of two songs.
I've heard remixes that a lot of them don't sound a whole lot different. One might be bass heavy, one might have strong bass and drum line for dancing, one might be instrumental, one might have a guest artist.
If nothing else everyone could play a guest track and then just mix it into their track using their finished mix as a guide. That's a new mix.
Go for it! You gotta take a banjo chorus.