For similar civs, could you use the texture from a different civ? (like if you are adding a cav to Spartan, you could use a Athenian cav texture) It might not work for all civs though

I like the idea, one of the hardest things with "new" units though is actors and portraits, so you don't confuse them with the original units.

Probably for all civs. Maybe the scout cav also have a big vision range

scout calvary for all civs or just rome? We had "hunt" cav in a27 which basically was faster cav, but weaker... good for hunting and ok for scouting.  Its definitely something to consider bringing back.  The whole thing about citizen cavalary is tricky though.

What do you think of a scout cavalry? They could build scout towers, maybe they are fast but low attack and resistance. Another thing for Romans: Milecastles or Fortlets. They already have army camps, so maybe that wouldn't be good to add