A number of total conversions has already been anounced and some are making good progress, from what I can tell. Among those are mods covering the Samurai-era and the Punic Wars. If those won´t fail for whatever reason you actually might be able to play as a Roman soldier.
I´m still rooting for a Dark Ages/Vikings/Anglo-Saxons mod, but I fear the subject isn´t popular enough. ^^