Mmmm in my opinion exist a very easy solution...hire mercenaries...if you lack of cavalry units you should recruit mercenaries, for thisa i said about implemented more mercenaries buildings and units andas more in the oriental regions for muslims and rus factions
If you are Norway and you have good infantry but lack of cavalry, pay the mercenaries! For example hire mercenaries knights( yes , i know that they are expensive but they are a good solution)
Same for cumans or other factions that lack of good infantry, why not recruit some mercenaries steppe spearmen?