There are severel ways too achieve this. These suggestions I think should apply to mainly AI but some of them also for player
Military ways:
Less good troops - Increasing the cost improve troops beyond T4 would be one way make sieges less successfull. IE, less steamroll. Militia mould be more than a tiny obstacle. They could cost influence to upgrade maintain when in a kingdom!
Weaker armies - Make it harder to create 1000+ armies. Can be achieved in many ways but the easiest is probably that clan realtion greatly penalized inviting them to your army.
Smaller parties - Ofcource an option nerf the base partysize for all parties.
Cheaper/bigger garrisions and militia - More troops to kill - less steamroll
Diplomatic ways
NAPs and truces - With a basic solution for negotiated timeframed periods of peace. Even a "bad" implementation, that completly stopped you from doing hostile stuff with a NAPed or TRUCED enemy would do as a starter to expand some real diplomacy
Alliances - A delicate issue that easly could go wrong and cause exactly the opposite. But a thrilling thought!
De-escalation
Dangerous war-mongering - What if it was dangerous to trigger a vote for war? What if king vetoed the war and you had to suffer concequenses for your stupidity with bad relations to come?
Other things to spend influence on?
Anti-Bandit parties - Small parties of improved milita troops led by a Sergeant semi- charachter(T5, simelar to bandit chiefs and caravan-drivers) that protects your areas from bandits and hideouts. Could 10 influence to start and 2 influence/day to maintain. Affects the area of one single fief. You can activate it in every fief with a governor precent.
Emisaries - Improves relation with other clans. Costs 10 influence to start, 1 influence/day to maintan and has a 40% chanse to give 1 relation/day.
Relations
King - > Vassal - For each vassal, what if relations mattered Bonus for good realtions, malus for bad relations
Vassal -> King - More delicate! A good relation should be better than a neutral but I dont want to penalize rougue vassals! There should be ways to thunnel realm- influence into your pocket! leading to....
Intrigue
I´d love an expanded solution for this - but a thin one would do! Here are some
New owner of fief - This feature is there but what if it was used and usefull? What if you make clans support usurping powerfull fiefs and trigger a vote where old owner is not qualified? Id like to see AI using this... And alot. The king could have a sheap veto but why should he allways use it?
Vote to exile clan - This feature is there but what if it was used and usefull? I´d love this, even if a ruler should very rarely accept it... Uless he is the initiater
Blackmail fellow clan - You blackmail a fellow clan in your kingdom(Where you are king/vassal or merc). They could accept and pay you X influence and lower your relation by 20. They could deny, triggering a duel(simelar to hideout boss fight) where looser pay 2x influence to winner.
Some of these would greatly improve the lenght and fun of the game! I liek the thought especially of lords have other things to spend influence on than voting for war and sniping fiefs!