Yes, the game certainly needs more arbitrary BS rules. In the next patch, forming Prussia as non-Brandenburg will throw you out of HRE, because "logic".
Kings can`t be vassals of emperor, even if HRE is much larger than historical, because, again, "logic".
And, ofcourse, everybody knows that if you grow powerfull within a realm, you will not try to take the reigns for yourself, you would try to destroy it, or leave it. Because, once again, "logic".
If there is a need for any kind of arbitrary rule, that could be that if a nation grows big enough to challenge the emperor, something like 100% of FL and income of emperor, there should be a civil war, members will pick sides, and the winner get`s the crown, while the loser is cut down.