When you dismount onto an enemy mount it should knock the enemy off their mount. This is relatively easy if an enemy mount takes damage from your dismount skill it also triggers the enemy to be dismounted.
This also plays nicely into the mounts move faster in your territory angle, as of right now mounts moving faster or slower in different territories means very little as if an enemy mount does catch you it cannot dismount you, and even if the player dismounts to deal damage they just gave up their ability to catch the enemy. However if you could dismount the enemy by catching up to them on your mount that change would suddenly make more sense, and as you would also have to dismount to achieve this you would also not then have the unfair advantage of having your mount available while the enemy is dismounted.(as where if it were a trap or trick you could use it to de-mount your enemy and then mount up yourself and run away...)
Also if this were implemented the movement speed difference when in your territory vs enemy territories could be tweaked to make it easier or harder to dismount enemies as well.
Full Counter is supposed to counter everything, why else is it called FULL Counter... Just like 100 blades should really hit 100 times so you instantly die to retaliation.