I'm a big fan of this but Persians are OP themselves with the immortals and elephants, so we can break them down into two civilizations: Persians and Parthians. The Persians include Sassanians and Achaemenids and Parthians only include the Arsacids. This way, they can add a lot of detail like a Persian scythed chariot that actually deals melee damage like in real life that they were central shock troops or Arsacids cataphracts (Parthian knights).