There has been attempts over the years, all below average as far as i am concerned. Praetorians, Knights of Honour, the aformenioned Imperial Glory, a historical battles simulator "Great Battles of Rome" and a few others iirc. None came even close.
I think that CA got already such a huge jump start on the competition with STW, that by this time that they have accumulated so much experience in doing it and trying different features, the others would really betaking a huge risk to try a few failed attempts in order to (potentially) catch up with them. Hence everyone stays on their turf, unless some miracle happens.
EDIT There was a game with very good tactical battles engine in the American Civil War theme/context, by a very small niche company of 2 people that kept refining their engine/scenarios. Rumor was that it was good for hardcore wargamers and prety realistic.
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