My vote would be for the Parthians and Isrealites, as
quality sets of the Gauls and Germans have already been made. A civilian set like the medievil england set would be good and why not yet another set of gladiators.
Why not all of the suggestions so far, slowly but surely over the next few years produce all of Romes's enemies ancients like Napoleonics are always popular. Most collectors, wargamers, painters, love this era, and the figures I believe would be big sellers. I know that looking at the posts on this thread that I would buy most if not all of the figures suggested if they were to be produced.