I disagree that a powerful march, or even march, is needed. There is usually no fighting, only a few villagers when such music is played, so why chose something more epic/... ? If you have ever played civ 5, you would notice that when you start a game, Peaceful music is played. Even if you play as the Mongolian hordes.
Also, I find it very biased to have one/(or few civs) with an epic theme. It's like making of that nation something heroic while the other's aren't (only country allowed to have very heroical is Jan Mayen or Ulm).
And yes, my country Belgium has for now a march as theme, but it is not heroic in any sense and sounds fanfaric. Which is probably the closest you can get to typical Belgium music tbh.