I'm only loosely aware of the look of ancient byzantine soldiers, could you perhaps find some pictures of what you think matches the current upgrade line? By googling it I just sorta come up with a bunch of romans for some reason
I guess it's not so much that they're not roman in particular it's the upgrade tree goes from looking like soldiers that belong in an army formation to knights who look kinda strange fighting in a shieldwall. I dunno... I was just really let down and surprised that the look of my army got totally ruined as I upgraded my troops.
I don't think there are any faction that has a more fitting.. heavily armored shieldwall type elite infantry is there? If there is I'll change exchange my legionaries for em... I feel kinda cheated that I don't get a proper shieldwall