Hmm, what are the gameplay obstacles? I'd think it wouldn't be very tough to design a succession system where nephews of the ruler interit
If I recall correctly (it was in the early phase of CK3, so few years ago), the main concern would be that this succession might be very hard to manage, both for players, and especially for the AI. Hence...
this could cause very easy game over, which - furthemore - can be very hard to prevent. In the first phase this could be relatively easy - you know who are your sisters, so you chose matrilinear marriages for them. but even for the player it gets more complicated then. Your heir is your matrilieal nephew, so to protect your dynasty inheritance, you need to make sure that his sisters are married martilineally... and so on. And now imagine that you have a long-living ruler, who can outlive many his younger relatives and you might need to take care of several generations of these successions, otherwise you can easily get a game over.
An easy and practical solution could be to set AI marriages of all female members of dynasty to matri-marriages. But that could make it lot harder to make any marriages, and from historical standpoint it would actually be ahistorical. It was this exact succession type, what enabled the Arabs to first take over the Beja people at the eastern outskirts of Egypt and Nubia, and then to seriously weaken Nubia itself.
So the only historically accurate way would be to require both the player and the AI to keep track of this complex line of succession. And I can understand that people may consider this to be too much micronamagement and too unfun.
I think we need to emphasize that this argumentation happened years ago, when the game was quite different from now... and it might be possible that now with adventurers etc. this might not stand as valid as it used to years ago...
I myself think it would still be worth trying and would be better than the actual setup, but I sort of understand why they don't have it as priority.
tl;dr - I don't think it's to hard to create this sort of succession type, but much more complex to make the AI to use it propperly and at the same time make it
fun for players. And as much as I myself would love if CK3 would primarily be a historically-accurate history-simulator, let's not forget that for the developers this is primarily a game, which is meant to be fun and that for them
fun aspect is more important than accuracy.