Advice from another Roman Campaign
I've got a long running High Fantasy campaign world set in an alternate late (currently post-) Roman Empire. My biggest advice would be to hide the 'historical' elements as much as possible. Draw as much inspiration, characters, ideas, maps, situations from real world materials, but obfuscate it. I've found that a whiff of anything either educational tends to cause player unhappiness (and my group has one person with a BA in history, and another with a Masters in classical studies, so they weren't know-nothings).
Spinning the map is a good idea, another is to restrict player maps to just the local area (100 miles in each direction or so) at least at the beginning. One useful resource was a non-english atlas (an old Romanian atlas, so much of the action has taken place in Dacia and Pannonia), that I could use to defamiliarize place names.
Some interesting questions are - is the Empire Good or Evil (depending on what that means in your campaign. In my opinion you can make a solid case for the historical empire as Evil). How has magic use affected the empire's military structure? Just having long-distance communication would have made a huge difference in the real world - much less the impact of mages and powerful monsters on the battlefield. Is the emperor a god, (as was the state religion between Augustus and Constantine)?
My mistake was to make the empire too strong at first, making it approach a modern bureaucratic state with lots of official magic users, and it was sort of dull for the players. So I blew it up - the time period after the fall has many more heroic options.
Pax,