One of the big problems with fighting zergs is that they pour on an outnumbered opponent by mounts and it's very little to do about it.
To disengage even with perfect CC managements is rough, as anyone of the attackers who is pushed off or falls behind can easily catch up by horse.
Spoiler:
It would also generally make engagement more risky, hopefully making a premade think twice about stopping to gank a significantly smaller force, as the reward might not be worth the risk of your entire team being on foot for the combat to end.
Or maybe something more drastic like having a minutes long cooldown on mounts, to force the players to make sure that when they are dismounting to engage in a fight, they are commited.
Even slowing mounts in RVR could make sense, but i would strongly suggest to buff movement speed in pve(pve mounts with 100% speed for example) so you could easily reach access points to RVR lakes easily and move slower from there on.
So this would maybe improve the game by making CC more impactful, making coordination in the region chat easier and more important as movements are not as fast or sudden, and making the fights hopefully last longer even less decisive when fighting a greater number.
I'd like to hear some input on these ideas