This is the one thing I would really like to see more of in the game
A trait related to knighthood for martial courtiers/commanders would be really nice, as well as something squires, pages etc. Basically something that gives knights a real presence in the European side of the game! Right now, all they really have is heavy cavalry in the form of levies and knights certainly aren't levies. They're professionals. Having cavalry/knight retinues will fix this but those retinues are still culture exclusive and even so, the cavalry retinue has about 50 HC and 200 LC i believe.
I would be very happy if knighthood, and anything related to the minor nobility could be added to the game. Something that would make those nephews, cousins and kinsman too far in the lines of succession to matter seem a bit more interesting to players
More events would be great too. There's only a few I can recall that have anything to do with knights. There's a grand tournament which can't be used too often, as well as events about hedge knights. I would like to see them have a bigger role in events especially battlefield-related. Knights are very interesting to me and I can think of many ways they would make the game feel more fleshed out for european feudal lords.
At the Battle of Bannockburn, a Knight named Henry de Bohun charged directly for the King of Scotland intending to kill him and end the battle. In CK 2 terms, Sir Henry was the nephew (or cousin, varying sources) of the English "Marshal", the Earl of Hereford and not in line to inherit. He was of minor nobility within the de Bohun family and he is hardly mentioned anywhere in history outside from his role in this battle. I think stories like these are very relevant here because it shows what knights and minor relatives within a family can do to make crusader kings even more enjoyable than it is!
TLDR; I agree with adding traits, events and seeing knighthood more heavily entwined with characters who aren't particularly in a position of power (courtiers, distant kinsman)
Just thought i'd express my thoughts because this topic has literally been on my mind for years concerning this game and I love Crusader Kings II. I'm sure something like this could be implemented without changing too much about the game