I can just agree. I started playing mongols because I wanted cavalry and horse archers. I stopped playing mongols because of the same reasons and playing RUS now.
At the moment its hard to talk about mongols since everybody is considering them as OP anyway but I would love to see some nerfs to their infantry and some buffs/focus to their cav.
While I dont see a new system with seperated horses it would be a cool idea and could go on further with abandoned siege which could be overtaken by infantry (yes, just like in the other Relic game - COH). BUT this is far too advanced and would change too much of the current game and I dont think it will happen.
Horseman and especially the Mangudai need some love (after nerfing YAM network for infantry): Make them way cheaper, increase the range of Mangudai (they are shooting the same bows as the foot archer but just a little over 50% of their range? Really?), and give them maybe another unit tech upgrade in castle age (similar to the RUS one with more HP for cav in general).
At the moment Mangudai are “meant” to be a raiding unit but even there other units just do a better job at it because they are so expensive and fragile.
Not long ago I tried to give this a special insight, maybe you are interested: