Don't get me wrong, I love the romans, which are my favorite ancient civilization (three main reasons: catholicism, latin alphabet, and cool military). However, while they would be the most fun realistic mod to play, they still wouldn't match a fantasy one well enough IMO.
Take Call of Duty, normally based on World War II. Now take Call of Duty: Modern Warfare 2. One is pretty realistic, while the other takes place in the near future in something that preferably won't happen. However, Modern Warfare was (IMO) far better than the former, even though it'll most likely never happen (controversal mission and all).
Another example: no comparion - Final fantasy. None of it's realistic at all, but damn, it's great!
Yet one more example: Movies. While there's a few good true movies, almost all drama, most really big movies are fake (LoTR, etc)