Well I dont think I have to explain too much, why I feel like cumans need a buff.
They have a pretty generic military with no economic bonusses, except for one that requires to follow a particular strategy that is easily countered on all semi open maps and still having below 50% win rate on Arena.
So to compete with other civs, you either have to simply play better than the opponent, or you have to go for the two tc boom, a boring, one dimensional, easy to counter strategy.
So my idea is to buff their early game without affecting their two tc boom, or late game power.
My idea is in the same vein, as the recent Saracen rework, which I actually quite liked. Take the power from a unique tech and redistribute between the tech and a native bonus:
Reduce the bonus from Steppe Husbandry from 100% to 70% training time reduction for Scout cavalry, Steppe lancers and Cavalry archers, and give the Cumans a civ bonus, that natively reduces the training time of these units by 30%. This way Cumans can execute a decent scout rush that actually has an edge over other civs scout rushes, with the fastest producing scouts in feudal age (23 vs 30 seconds).
I don’t think it gives them a huge advantage (its not supposed to), but it gives them a slight edge that should enable them to do something else than just the two tc boom.
I could also potentially see to extend the civ bonus (not the tech) to affect camels as well. Cumans do not get heavy camels, so they are not a unit they want to go for in the long run anyways, but reducing the training time from 22 to 17 seconds for them might give them a nice boost in early castle age as well.