I completely agree.
I also hope that the difficulty option in CK3 drops the opinion malus, and instead boost AI aggression towards human player, boost their ability to amass larger levies, and boost their anti human alliance, especially if human player is becoming a powerful Empire.
Making your vassals artificially more angry with you isnt that fun in my opinion, though, a little harder overall wouldnt be a problem.
Also, make religion opinion modifier harsher, and stress a lot more stressful. As of now I havent reached stress level 2 more than once when I first started out.
This game needs more external threats, and mechanics to deal with powerful Empires, especially human empires. Right now it seems that the AI(Which in spirit of RPG should care about the world around them) is oblivious to rising powers, and the islamic world keeps loosing to the crusaders.
One more thing, Paradox said they had mechanics for making smaller nations able to defend and maybe win against larger nations, but the battle modifiers are so soft that the biggest number still wins. I never think about terrain, passing a enemy castle might kill 150 men, so what, no battle modifier malus for behind enemy lines? Supply wont reach you that easily behind one or two castles.
Make medieval castles shine, they were a major slow down to warfare and immensily important in medieval times. I would give few exceptions to this rule, giving nomad horse peoples, like mongols and other steppe people able to raid and pass enemy castles without malus modifier, making them bad siegers, but a dangerous raider and open battle army.
Okey this became a rant, and its all written in a hurry, so take what I say with a grain of salt.