Why not just one cavalry class which you can specialise either for combat or scouting/healing. Currently cavalry units role is kind of limited as they have hard counters (pikes/anti-tank). So giving them an extra role would increase their usefulness.
1) In my suggestion, counter units would drop out. The line was always a bit strange with a lot of shifting/blurry roles (e.g. fast/slow) and an overall limited popularity. Spears and Pikes would become the standard, ressourceless early infantry, while lancers and gunships would be used for lategame light infantry.
--> Heavy cavalry would surely not be less desireable.
2) Diversity is fun! The two cavalry lines would have massive differences and totally seperate roles.
3) If we use a ressource (horses/oil) for two unit lines, we could make it more common on the map (less likeness to get none at all, better balance), but it would still be rare and precious.
4) Some promotions associated with scouts/vanguards could be very overpowered if allowed on strong, elite combat units. Some combinations of promotions can be overpowered. Separating the allowed promotions on 2 unit lines avoids these problems.
5) It is easier to teach the AI how to use units if a type of unit has a clearly defined role. If a certain type of role is only available with carefully selected promotions, the AI would have a hard time taking the right choices (So would inexperienced players).
6) Scouting knights or tanks seem weird.
7) If we have visually distinct light and heavy cavalry, you can immediately see the role the particular unit has. It is tiresome to always check the promotions to see which unit is your scouting cavalry/tank and which your combat cavalry/tank.
To sum up my suggestion in one sentence:
To replace the counter line and the vanguard line with a light cavalry line that is non-spammable (uses horses/oil) and is designed to avoid combat, not to be part of the main army.
Combat roles: Scouting and recon, eventually healing.
Upgrade paths:
Combat role overview:
(I didn't seperate iron and non-iron infantry in this overview, but the ressource units would surely be better at offense)
For comparison, Thal's original version: