Well, I'd personally rather see the Hippus regain some of their Wind-knowledge. Like some elder units (aka archmage) with shining blue-white eyes, and whipping wind all round him (robes and such flying about). As well as granting Their Knights Air II spell and renaming them Windriders.
Also perhaps granting either Air II or Air III the "fervorous wind" ability/spell, which acts like an extra mobility to all arcane-class (and mounted class?) units, as well as an extra 10% withdrawal. Also perhaps give them a National Unit of Pegasi which require Warhorses, Strength of WIll, and Omniscience, who are Flying, have Air II and Air III, and can be upgraded from an Archmage or a Knight.
Heck, these Pegasi could even be the Hippus UU for immortals with a different tech requirement and unit upgrade, however it would perhaps be OP to allow Archmages to upgrade into a flying immortal unit, so maybe only mounted units would be able to upgrade into the immortal and flying pegasi.
Perhaps even granting the Archmage the title of "wind lord" with flying promotion, ability to buy flanking promotions, as well as +1 air affinity.
Although, on Second thought a WindLord sounds like a good Druid UU for the Hippus, with +2 air affinity, Air I, II, III, flying, ability to buy flanking promotions, and 35% withdrawal. Or make him a storm lord with +1 air affinity, +1 water affinity, Air II, III, and Tsunami.
If you did a coding system where different improvements spawned units ... then might I suggest Mines-> swordsmen, Cottages-Towns -> Assasins, Farms-> Horsmen, Pastures->Horsemen/HorseArchers, Plantations -> Hunters/Rangers
tech dependent of course. And if they are going to be a Nomadic or non-city civ, then might I suggest that all units start with mobility1 and flanking1.
//Is it possible to get Drill affinity? I'd like to know if you could make all HIppus cavalry get an extra first strike chance per air mana owned.