Some of these things do exist in game.
You can donate troops and prisoners for influence gain - this is done in city and castle keeps. You can help other nobles in battle. There are quests that involve clearing bandits that give you relationship and loyalty gains with the notables and towns that initiate them - look out for them in cities and villages - they're an important part of managing cities, and there are reputational impacts between characters who dislike each other. Even when factions are at war, if clans and characters like each other, they're less likely to target and fight each other and visa versa.
Non aggression and trade pacts would be interesting, but I do know a lot of work is going on regarding diplomacy at the moment, the next update might see some changes there.
The stances seem very Total War and I think there is resistance to making Bannerlord more Total War.
Regarding faction specific units... this wouldn't really be 'historic' for the periods that the game is influenced by. In the medieval period, armies weren't ethnic or nation specific. They were either professional companies fighting for anyone who pays, or they were feudal levies fighting for whoever was the local lord no matter what their ethnicity. For example, a French army of the middle ages might include Flemish, Dutch, English, Scottish, Spanish, German, Swiss or any multitude of forces. Byzantine armies could be even more cosmopolitan. Even Turkish armies of the medieval period might include Norman or Italian mercenaries fighting along side Greek or Armenian soldiers levied in Turkish ruled cities. Cosmopolitan should be the rule.
The same is true for armour. The best armourers and workshops were famous across Europe. Milanese armourers would work for English lords, German armourers would work for Hungarian lords. Certainly there were localised variations, but there was nothing to stop the King of England buying a fancy suit of Italian armour. It could and did happen - as evidenced by what you find in the Royal Armouries Museum in Leeds.
I do on the other hand like faction specific goals and quests...