"modern tune - that doesn sound folkie :(, that Rozsa could be danced to?"
What's wrong with the eponymous tune <-- (look! it's a link!) referred to by my esteemed friend in an earlier message?

As Malcolm says, any slinky (or non-slinky) waltz will do, or something that sounds waltzy (observing that the above tune is notated in 6/8)

Perhaps I should have a go at writing something with a Balkan/middle eastern flavour to it (on an Armenian or Macedonian scale of the sort whose key sig has both sharps and flats, for instance). I was thinking of doing something snake-charmer-ish for Pig Dyke Molly but I could make it a waltz instead...

I think at different times we've perpetrated all of our usual waltz sets on dancers doing Rozsa: The Shrewbury Waltz, Princess Caroline's, Tommy Bhetty's, The Cumberland. Possibly some of Nick Barber's too.

The Blue Danube wouldn't be quite right.

Anahata