I thinkthat's a good and necessary decision. I have struggled with the same issue in my own planned overhaul mod: how to make playable a large and influential nation that was in reality completely non-dynastic?
Understanding that it is no priority right now, I do feel like pondering which succession law would be most applicable to the not-historically-dynastic-but-dynastic-anyway counties. One might suggest to add a layer of pseudo-realism, with a custom law. Primogeniture would make playing as a Chinese count too easy, and gavelkind would be ahistorical as much as nonsensical because elective/administrative entities should not just split up randomly when the current official deceases.
I imagine, to at least give the impression of probability, to have an elective law that chooses from amongst every gender/age-eligible member of your dynasty, and whichever courtier is most popular (to be determined with a basic scripted score value that looks for traits and e.g. diplomacy value). The idea is that you, the player, are doing such a good job as an elected/appointed official, that you can convince them* to elect a dynast when your time comes to retire. And conversely, when you have not been doing a swell job (using many of the same Mandate of Heaven factors if possible), you will make the popular courtier, who is a representative of "any candidate but your dynasty", swell in votes. That way there's still a factor of danger when playing as a realm that is not supposed to be dynastic, while not rendering that country completely unplayable.
If one were to add such a law, I also think the AI should not use it, meaning only player counties are ahistorically dynastic while the AI counties just generate new rulers out of thin air. That way you cannot form long-term marital alliances with non-dynastic counties either.
As I said, it is most definitely not going to be a priority, but I thought my musings might be interesting at least.
* Them in this case being your own courtiers. You could give the vote to the Emperor instead, but I imagine forcing him to choose for a successor for every single county in his realm to not be very fun to play with. So your likelihood to have someone of your dynasty appointed after your death would have to be represented with popularity amongst your peers.