I don't play too much international music, and I'm not crazy about the styles and voices in the international expansion packs that I had put into my S970 - I tried one that had a few Irish styles and voices and one that had some Mexican styles and voices.
I wish that Yamaha came out with an expansion of a genre - like a great set of jazz and swing styles with killer brass and woodwinds, or country, 60's or 70's pop, R&B, ballroom. That's something that would be exciting to put into the keyboard. I'm pretty happy with the voices already, but it would be fun if the expansion material sold by Yamaha had some of the S.A. and Megavoices that are in the Tyros 5 and Genos but didn't make it into the SX900. I'd pay extra for that.
Anyway, so far, my expansion slots are empty.