Something mainly arboreal would be good. Mainly because you'll see them a lot more in a display cage like that. A good GTP or ATB would look lovely. You could also divide it in two and put in two smaller arboreal snakes.

Carpets need a bit more floor space IMO, same with RTBs. I think an RTB would be very hard to do in a cage like that because reaching in to pull out a heavier snake could get difficult and slightly dangerous.