Massive post incoming. Ten points. Not all are strictly cav v cav, but when poking a stick at a spider web, there's always a wide range of changes.
Maximum lance angle for a stab should be dependant on lance length. Longer lances should be restrained to the forward arc. Shorter lances should allow stabs to the side, at least the left side. A very short lance like the light lance could even allow a stab to the left rear quarter. This will result in lancers having to decide if they want a long reach but narrow engagement area, or a short reach but wide engagement area. Short lances would be effective in horse-to-horse melee.
The great lance should be one slot. Why is it -not- one slot? You only ever have it in your right hand. When it is not in your right hand it is discarded on the ground. You likely didn't even carry it to the fight yourself, your squire lashed them to a pack mule. This would let a greatlance/1h/large shield knight proper start playing. Before you freak out, remember lance breaking could come into effect.
The idea of lances breaking after so many couches is an appealing balance consideration for when lancers begin to thoroughly dominate through the use of couched lances. Right now most lancers I see use the heavy lance and stab it. Couching proper is rare. However, should lances start taking damage like shields do, we need to consider the circumstances. Lances are polearms. Most 'lancers' use their lances in thrusts rather than couches. Nothing would prevent these from switching to warspears or otherwise to avoid damage to lances caused by non-couched attacks.
Couching doesn't really need a nerf, which lance breaking certainly would be. If unsheathable and couchable lances became one slot items, perhaps they would. But right now one can only couch when charging straight at a target with terrain favorable (and no pikes around), so breakable lances might nerf couching into the ground. Lowering the 'speed limit' for couching would help.
Arrows need to do a flat 50% less damage to horses, or the hitboxes need to be adjusted, or horses in general need far more armor. Lets face facts; most of a horse's head is sinuses. Only the area behind the eyes is the braincase. An arrow has to go through two or three times as much meat and bone to reach an organ in the thorax. The entire back of the neck is a muscle. The flanks could be riddled with arrows before the animal dies of bleeding to death. Honestly, you should not have a reasonable chance to kill these animals outright without the sort of force that would completely impale an armored man, and even then, only to a weak point. The practical result of some increase to a horse's durability would be more focus on killing the man in cav v cav, which is currently more like man versus horse and then see if you can finish the rider off conveniently. It will also make melee horsemen viable starting with a horse like the destrier.
Although this is nominally cav v cav, cav v inf is an interwoven theme with every change you make; nothing happens in a vaccuum. So while I'm on the subject, hitting E while my horse is moving slowly should give a rearing animation and two big blunt-damage knockdown kicks in front of my horse equal to the charge value.
One change needs to be made regarding horses; a horse that dies needs a quarter or half second of 'momentum' time before it falls over and the rider dehorsed. This means a thousand pounds of horseflesh, killed with a non-staggering blow, just before hurtling into a mass of men will behave like a thousand pounds of horseflesh crushing puny infantrymen, rather than cotton candy. This would put more emphasis on staggering horses when you kill them, as it would prevent the bump-damage steamroller. Outright killed horses with a stagger would rear up before collapsing. This has a particular effect on cav-versus-cav lancing.
A couched charge into a cloth archer shouldn't damage the lance. A couched charge into a peasant's old heater shield shouldn't damage the lance. Basically, a couch should have to be successfully blocked for the lance to be damaged; high level shields, great shield skill, weak charge. If the shield breaks, the lance only takes partial damage. If the shield survives, the lances takes full damage. How much abuse a lance takes should depend on its length/weight. Heavy long lances should be able to take more punishment than light long lances, and light short lances should be more durable than their longer counterparts.
When couching with a shield, the shield should be blocking. Then we would finally have proper cav-on-cav head-on jousts! The challenge would become hitting the enemy player directly or his shield and shattering through while he does the same. If he aims for the horse instead, the horse will continue for a half second as the previous point suggests, and the lancer while dehorsed in a moment can still kill the enemy player. This will encourage aiming for the man rather than the horse in head-on lancings. Furthermore, right now archers can line up shots on a charging lancer and hit them, foiling the couch. With the shield properly blocking, arrows will have a much harder time finding their mark. This weakens Xbow cav, who use this to defeat a charging lancer with a cheap staggering shot. This also makes archers more dependant on pikemen to defend them, since they can no longer rely on their own ability to kill the horse or stagger the rider just before the impact.
And lastly, as some other mods have, damage to the horse should reduce its speed. Starting at 50% or 66% damage, the horse needs to start running slower and slower until a near-death horse is barely able to outpace a man. This will make horsemen less effective the longer they stay in combat, reducing the rounds that end up with a near-death horse bearing around a xbowman at full trolling speed. It also counter-balances all the various 'buffs' that go into cavalry players, and makes cav-on-cav pursuit much more interesting. A good first hit might make the enemy courser slower than your destrier, while his first hit might not damage your tougher horse enough to slow it.
To summarize.
1. Tie lance stab angle to length of lance; longer lance, narrower angle.
2. The unsheathable, couch-only lance should be one slot.
3. Couching damaging lances must be created with other changes or else it is a flat nerf that will drive people away from the class.
4. Lower the speed limit for couching.
5. Increase horse armor, total hitpoints, adjust hit boxes; just make them tougher to kill than their riders!
6. E should rear the horse and send two big knockdown kicks in front of the animal with some blunt damage to boot.
7. Horses should not die immediately, rather persist for 1/4-1/2 of a second before dying.
8. Only a successfully blocked couch should damage the lance; a broken shield damaging less than an intact shield.
9. Couching should activate a shield's full block potential.
10. As a horse takes damage, it should slow, starting at 50%-66% health on down.
Points 1, 5, and 7 increase a horseman's close quarters capability against cav and infantry.
Points 2, 5, 7, 8 and 9 combined will result in head-to-head jousting between two couchers who actually aim for the enemy shield.
Points 1, 5, 6, 7 and 9 increase the horseman's melee staying power, as opposed to hit-and-run fighting.
Point 10 provides a needed counter-balance to cavalry.
I think that combined this would be a big change for cavalry and make them more complicated to fight as, with more options than the straight lancer we currently see wheeling about. Real knights that charge a horse into the infantry's main body directly, couch, take an arrow to the shield, run a 2hander through with their great lance, drop the lance, draw their sword and start hacking and kicking the horse until the beast is slain or the knight falls. Meanwhile light cavalry lancers could intercept these heavy knights and plague them with light lance or double-sided lance stabs at angles the heavy knight can't react to. Much more durable horses means archers will need to stick to the pikemen and pikemen will need shieldmen to finish off unhorsed knights. And horses slowing as they damage serves as the big balancing nerf; horses wounded in battle become less effective, meaning all cav, but horse archers and xbow archers in particular, become less dangerous the longer they're in battle.
