So, I think I figured a way to make the player care if they were cuckolded. Because as it is now in CK2 the right solution is always to trust your wife, even if half your children were unexplainably black. And it never felt right.
So how can it be solved? Of course, the most obvious solution is game over if you ended up not really having genes of your original dynasty. However, as obviously, that would be too harsh and random, but then if you remove the randomness and make the player always know which child is theirs and which isn't, it's no fun too. Although, as I mentioned before, there should be more control over it, but 100% control also removes the fun.
So what I'm suggesting instead is: on succession if it happens that your oldest son is not actually yours, he still inherits (because everyone around think they are), but you as a player end up playing the closest dynasty member who is its member in actuality and not just belief [among the landed ones for now, but since they said unlanded gameplay is not impossible in the future, imagine trying to retake the throne from an unlanded position]. This then makes it actually important to give land to your sons, since you can't be 100% sure which of them you'll end up playing as. And then when it happened you can try and regain the throne by pushing your claim, and you can plot against the main heir to discover and expose that they are a bastard, which should give them a large enough opinion malus for more vassals to back you in your faction.
So how can it be solved? Of course, the most obvious solution is game over if you ended up not really having genes of your original dynasty. However, as obviously, that would be too harsh and random, but then if you remove the randomness and make the player always know which child is theirs and which isn't, it's no fun too. Although, as I mentioned before, there should be more control over it, but 100% control also removes the fun.
So what I'm suggesting instead is: on succession if it happens that your oldest son is not actually yours, he still inherits (because everyone around think they are), but you as a player end up playing the closest dynasty member who is its member in actuality and not just belief [among the landed ones for now, but since they said unlanded gameplay is not impossible in the future, imagine trying to retake the throne from an unlanded position]. This then makes it actually important to give land to your sons, since you can't be 100% sure which of them you'll end up playing as. And then when it happened you can try and regain the throne by pushing your claim, and you can plot against the main heir to discover and expose that they are a bastard, which should give them a large enough opinion malus for more vassals to back you in your faction.