So I was the king of Germany, France, Aquitaine and Lotharingia. I died and the game gave my first son France and Germany and my second son Aquitane and Lotharingia. This seems absurd.
Please let gavelkind rulers choose how their lands are divided amongst there heirs.
Also cadet branches.
Also let us intervene in are vassals war. One game I was Emperor of Rome and I gave my son the kingdom of Anatolia, and I couldn't do anything about his cousin King of Croatia declaring war on him for Anatolia. This is also pretty absurd.
Dude, we have to clarify some things, some of which have already been extensively discussed>
1. You mistake "Gavelkind" for a "hey, my firstborn, have this land. Hey my second son, have this land. Hey my third son." It's not like that. Read this
https://en.wikipedia.org/wiki/Salic_patrimony
The "what goes to whom" was not an unilateral decision from the ruler by any means, and, if the game were to be true to the real salic patrimony, there would be NO gavelkind, ONLY Elective Gavelkind. You CANNOT and SHOULDN't be able to choose "what goes to whom".
Besides, unless your demesne is REALLY fractured, gavelkind is kind of predictable, so you should be able to divide the land however you wish between your second, third, a so forth sons. In your specific case, why do you think the result is absurd? You have four kingdoms and two sons. That each gains two titles is only fair. And you have to take into account the extent and power of each kingdom, gavelkind does not divide titles equally only in name. If i'm not mistaken and you wrote in that order for some reason, France must have been your primary title, which is invariably inherited by your firstborn and whose capital duchy is preserved whole unless you do not posess land outside your de jure capital duchy. If you had two duchies before dying as one would expect, then your firstborn got France (your primary title) and your capital duchy, your second Son got Aquitaine (your second most powerful kingdom) and your other duchy. Then, in the next round of inheritance, your firstborn got another kingdom and your second son finally got the least powerful kingdom.
2. I don't know about cadet branches. It drastically alters the way the game is played. Imagine, for example, your dynasty had thousands of landed members, but you yourself only had two sons. If you had primogeniture and gave land to your second son (making him change dynasty to a cadet branch), and your firstborn died in an accident, your second son would invariably inherit, which would effectively mean game over even though you still have thousands of landed dynastic relatives. I don't know, i would love cadet branches, by i don't think there's a good way to introduce it into the game with the current mechanics.
3. Dude, you have a way to intervene in wars between vassals. Enforce realm peace, or get to medium crown authority to end wars between vassals. Besides, a claim is no simple thing, specially a strong claim. It means a legal right to that title. That your son's cousing declared war for his claim is nothing absurd. If you don't want shit from your vassals, you have the means to deter aggression, even if that means imprisoning someone and getting tyranny (or kidnapping and getting none). And remember that now you are the emperor. But what would you think if your liege suddenly started fucking up all your wars just because? I remember alright how many of the people complaining that you have no way to completely stop wars with Conclave, but became super frustrated once their lieges got Medium Crown Authority pre-Conclave.