Voted for a 500 5.6 IS, i wouldn't mind DO, unless price doubled ($2500 for a DO IS would be OK, anything higher makes me scratch my head, and go for a 300 2.8 with 2xTC).
Another great one would be a fast cheap prime, like 24 f/1.8, for $400-$500. Alternative would be a 20 f/1.8 and a 35 f/1.8, as a set, both for EF-S, which saves cost. If f/1.4 is affordable (Sigma manages, and i'm already looking at them), then make it faster, but i fear they won't, because of the L lenses, with their 1500+ price tag.