I would be interested in every ancient/medieval topic.
Sometimes there are new surprising sets/eras, I had ever thought ... and afterwards I like to collect and buy.
One big gap is (already mentioned) Byzantines (e.g. early = Justinian, middle = around 8/9/10 th century, later and late ...)
I'm still hoping to get Roman (imperial and republican) command sets (even big box 907 provide some nice figures). But with the large amount of Imperial Roman Sets I would buy command sets in a large number!
... and what I also like to see is an expansion of the punic war range with hellenistic troups (Macedonian (Philip V), Seleucides (Antiochos III), etc., infantry, cavalry, elephants, ...)