There are some interesting possibilities. My money's on us getting a track from The Nylon Curtain or newer, since the earlier albums are pretty well covered and are pretty heavy on keys... not that that stopped Harmonix from releasing "Always Something There to Remind Me" and "Africa". But if I'm right, then it will depend on if they're going looking to improve the catalog for full-band players or particular instruments. Vocalists would be best served by "The Longest Time" or "Uptown Girl". Guitarists, I would say either "Allentown" or "Keeping the Faith". Bassists would also have fun with "Keeping the Faith", and I think drummers would get the best workout from "Uptown Girl".
If it's for the whole band, then I'd have to guess "Allentown", as much as I might kinda-sorta don't want it be.