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Thank you guys for all of the responses! So it seems like Romano-Byzantine ideas are pretty dominant. I know Orion tried to do three sets of Byzantines, but they did not do well with shield poses in my opinion so plenty of room for improvement there. Some Late 1st Millennium AD Picts/Irish would be interesting too and would go really well with the Strelets 1066 line, as the 11th-12th Century AD Byzantines would. I am up for some Iberian and Italian Medievals as well; I have this image of a little Francesco I Sforza running around the apartment causing havoc.
My main Roman themed idea that has not already been mentioned is a set of 1st Century BC Gallic Auxiliaries in Roman service. They are mainly intended for Caesar's army and would show traditional Gallic clothing with some Roman style equipment types mixed in.
Linear-A have recently updated their upcoming sets lists (normal and mini sets) - byzantines are on the way. Set #039 is Eastern Roman Empire 6th Century (so in "early period" of the byzantine empire) - Roman Infantry Set 1, and #011s Imperator Justinian & Imperatrix Theordora