With a little help from the dot-codes and the feel of the shield, I managed to find three Roman Legionaries in one go - and then couldn't be bothered to keep hunting...!
Now, has anyone noticed the standard-sized hole in the top of the Legionary helmet, which is clearly set up to take a helmet plume? Given the tendancy to include several related minifigs within a number of series (like the Egyptians, Space theme, Sports theme, etc.), my guess is that we will see further Romans in subsequent series and I suspect the likely possibilities will be:
1. a Centurion
with transverse plume on a silver/steel helmet the same as the Legionary
and mail-style armor rather than the segmented armor, plus oval shield and gladius sword (+/- cloak)
This seems the most likely.
2. Legate / Tribune
some sort of senior officer with a fore-and-aft plume on a gold colored helmet and breastplate armor,
plus gladius and cloak. This might end up looking similar to the Spartan but still seems reasonably likely.
3. Signifer / Aquilifer
Similar to the Series 6 basic Legionary but set up as a Standard Bearer with the unique acessory of the classic Roman Eagle insignia.
The printing used for the robes on the current Statue of Liberty also makes me wonder if we'll see something similar used with a white body (maybe purple/gold edging...?) for the toga of a Roman Emperor, perhaps with a unique laurel-wreath head-piece?