I want to say 1 thing about diplomacy, the option of threaten the other kingdom and try squeeze them some money or peace was a bit bug on diplomacy, when u had a new kingdom with no strength and u send companions to squeeze the other kingdoms or peace all fall in ur petitions almost always... was in a old revision of diplomacy idk if still like that...
good things can be added kind of diplomacy stuff or reworked can be: alliances, help in war, terms of surrendering or peace (can be fiefs, castles, etc), can send companions to persuade lords to join u and assistance that tell u which lord have no fief (edited: this idea was from this mod LOOOOL nice
).
p.d.: im new in this mod and idk how it is the option of persuade the enemy lords when they are prisioners, hope it works a lot! but the options of send companions to persuade lords can be complementary anyway, in real life kings always send diplomatics to speak with other lords they didn't travel the world to convince them.
edited: yep im playing brutality mod and i just send peace to the Dark knights and they accepted with no demand, when i took the castle from them and i was with not so many men defending.