Hippus are pretty powerful as is, and frankly I play them pretty much like Genghis Khan was their leader... zip in with mounted troops, ruin everything you can, maybe take a city or two, and just keep doing that. The horselord promo they get is really, really good, but it isn't very useful if you don't attack things, is it?
If considering additions for the sake of "flavor", if that flavor also results in some kind of increment in terms of power, it might need to have some level of detriment accompany it. Could even be something extreme like weakening or otherwise limiting some of their non-mounted line, like if their non-mounted tier 2 & 3 units were somehow gimped, reflecting that the cream of Hippus warriors are on horses, everyone else is not made of stuff as stern. Silly example might be that Champions wouldn't get +25% vs. melee, but they'd get +25% vs. mounted, since as the "horselords", who's going to have the best grip on countermeasures against a mounted assault? Give them pikemen graphics if you wanted, in such case. While this means meeting another stack of enemy Champions in the field would see a disadvantage if going toe to toe, Hippus tactics would dictate that a terrifying stack of horsemen, horse archers, chariots, whatever, are used to tear a stack of enemy Champions up before other of your "less elite" forces come in to try and mop up.
Anyhow, yeah, I guess all I'm saying is that if there's opportunities for gold, or extra merc. events, or other "fun and flavorful" mechanics, it might mean having to whittle away in other areas for balancing reasons. Even if not at the unit-level, there's got to be other creative ways to achieve balance, like leaders all getting some extra not entirely positive traits (maybe make a new version of the Greedy trait called Mercenary with some gold / unitclass based xp bonuses, and a variety of other malus for say, diplomacy, happiness, maybe a research penalty, a promo on your units which causes them to sometimes get the Enraged and Bounty Hunter promo at the same time, going off and murdering someone for gold, ha ha)