What I suggest is you having to dedicate alot more to riding in your build to use any good horses and to that end I think these are more fair Riding Requirements for horses:
Sumpter - 3 Riding Requirement
Rouncey/Palfrey/Steppe/Desert - 6 Riding Requirement
Destrier/Arabian/Courser - 7 Riding Requirement
All the Armoured Horses - 8 Riding Requirement
I don't necessarily disagree with the spirit of your suggestion, but I do want to point out that the requirements for riding you've proposed aren't likely to be approved because, as Malaclypse pointed out, they aren't parallel with the other requirements in the mod. Consider:
-If you are melee, 15 strength is enough to use a plurality of good weapons (the Longsword and its variants, most of the greatswords, Morningstar, Barmace etc). If you go up to 20 strength, you can use every melee weapon in the game.
-If you are an archer, 15 strength and 5 power draw gives you access to a good bow that is also the most commonly fielded bow in Strategus (Horn Bow). If you go up to 18 strength and 6 power draw, you can use every bow in the game.
-If you are a thrower, 15 strength and 5 power throw lets you use all of the best throwing weapons out there (Heavy Throwing Axe, War Darts, Jarids if those are your thing). You can go up to 21 strength and 7 power throw, but there's little point in doing so as the Throwing Lance is gimmicky and expensive.
-If you are a crossbowman, 15 strength is enough to use every single crossbow out there.
I assume this is why, for cavalry, the developers decided to go with 15 agility and 5 riding as the requirement to use most of the good horses and 21 agility and 7 riding as the point where you can use every horse in the game. If you want to get them to increase the requirements in order to make hybrid cavalry less viable, you may want to address why your proposal is better than some of the other changes people here are suggesting that would have a less deleterious effect on many existing builds.
