I would love to hear them do a Beatles song one day... And I Love Her, Eleanor Rigby, I'm Looking Through You, Nowhere Man, Day Tripper, or Here, There and Everywhere.
I would also love to hear them play Cold Hands, Warm Heart by Brendan Benson. And it would be incredibly cool if they would cover a Coldplay song one day: IMO Tom's a better singer than Chris Martin so that shouldn't be too difficult. ^^
Oh, and of course a song by mr. David Bowie. I would love to hear Sound and Vision. And I think Tom should try and sing lower more often, so why not do People Are Strange by the Doors. I'm sure Tim could give that song an 'update' for the keyboard parts. 