umm all of them

seriously: Ceaser would at least in part tie in with sucessors so would need to be written eith that in mind (a lot of cross over)
Legion: interesting as there are not that many significant enemies (Germans, Dacians and easterns, plus other Legions) that did not cross over into other supplements.
greeks and persians overdue.
Trojan war raises the idea (which i have thought would be good for a while now) of Chariot wars redux, being based on the main periods of conflict (trojan war, quadesh, rise and fall of assyria) to expand on the work already done.
of course all well written WAB supplements will be appreciated.
Ben