I'd like to see what they could do with arranging POR/RD tracks*. Them doing a take on Against the Black Knight would be incredible.
*Or any Gamecube game, really. The only GC arrangements I remember in Brawl were Luigi's Mansion, Legendary Air Ride Machine, and a couple of Pikmin songs.
Tellius arrangements would be good, but the at least the synth of the originals still sort of holds up. I'd prefer some more pieces to be arranged from Gaiden or the GBA games, simply because of their tendency to be overlooked as well as the desire to hear them freed from the constraints of the NES and abhorrent GBA sound hardware, respectively.