Grand Alliance. Set it up like George Orwells 1984 so if the player or faction X hits Y power the whole world allies against you and just rages against you with giant armies made up of several factions that suffer near zero debilitating problems associated with. Have them just hunt your hind end down, lay waste to your cities, your lords become flighty, said factions use less fodder troops and take war seriously and send well trained, professional forces. They stick together, no picking them off, think Napoleon's "three wings" that converge the INSTANT you initiate a fight, no waiting, NO WAITING, the other two armies just DIVE on you. They are also hard coded to stay within close reach of each other. Mercenary forces flood to join them because they are paying better than you are, said mercenaries then begin just a wave of terror and destruction across your entire kingdom dividing your time and efforts, they just burn and pillage anything not nailed down, wipe out all peasant groups, caravans, etc. Totally crash your economy while the main thrust just slaughters your cities and garrisons.
As someone else said: Assassins just EXPLODE in number, everyone and their mothers uncle wants you DEAD, getting wounded by an assassin means not just 1% HP but you are bedridden for days or weeks and unable to command anything or organize defenses.
Allied army(ies) dont waste time on your "fluff" cities that produce diddly, they aim for your core, the most prosperous, precious cities and then grind down the rest later. If you lose X amount in a short time lords rebel and join them taking their fiefs with them, peasants mass rebel and cities go up in flames as you are proving to be a poor leader who can not protect them and they do not want to die. All and I mean ALL of your trade has come to a total, absolute halt at this point, allied trade triples and they are rolling in denars which means more mercs and bigger armies or more of them. Your own merchants lose faith in you, turn on you, open city gates for your enemies so they just walk in, no siege necessary.
Bandits explode into existence, your troops are MASS deserting you, turning into the old Warband groups of 76 Khergit Lancers and such that tear apart everything. People are even more on edge, economy in death spiral, you cant make money, no one has money so you cant "smith cheat" your way into staying afloat if you want some hard-core fun that way. Cities maybe have... 50 denars? At this point, bandits and enemy raider mercs are just mopping the floor with your peasants. Not a single village is NOT in flames. Your people starve, your army is joining the enemy, you have no food, your own family wants you dead now and god forbid anyone captures you because it means INSTANT death. Oh and main-char death does NOT mean the war is over! They hunt your family down, your children, that is the way it worked back then, leave no stone unturned. Main-char beheaded, eldest sons and daughters dead or sold into slavery... Your new-char is running for their life or trying to hold on.
That should spice things up a bit. Let the AI cheat like an SOB to when it goes "Grand Alliance 1984." Give it a nice "we are the Germans, but we dont even have the Italians or Japanese to soak up some of their bullets for us" vibe. A nice, "Siege of Berlin" kind of situation except it is not just the Soviets but EVERYONE pounding down the gates. AT THE SAME TIME.