A Roman theme would have a lot of different unit types. First you could have legionaries with Lorica Segmentum (the classic plate style armour), then a different print with Lorica Hammata (chainmail) and a third print with Lorica Squamata (scale armour). You could also include different unit types such as the various types of auxilia used by the romans (including a wide variety of cavalry). You could also differentiate between ranks with various types of helmet plumes and crests. You could even go for an earlier Roman period, that of the Republic pre-Marian reforms and have Hastati, Principe, Triarii and Velite, or you could go for the late Roman period and have late style legionaries and foedarati (barbarians employed by the Romans).
For enemies you've got the Greek city states with Hoplites and pikemen, Parthians with their very heavy Cataphract cavalry, various barbarian tribes and of course Carthage which should include elephants (I'd probably go with Carthage). So there is plenty of scope for interesting mini-figs.
The buildings are somewhat harder but could include a roman fort, town gate (being besieged), an amphitheatre, regular theatre, a temple, a roman market street (based on the one at herculaneum). You could also include galleys, siege engines and other cool stuff like ballistae (flick-fire time).
The Trojan War could also be a cool theme as its all myth and legend anyway (I don't think there would be a need to edit out the gods of religions that have been dead for 1500 years).