Adding cadet branches seems like a great, but it will require certain rules. Certain rules will be needed. Some look useful to me, especially rules concerning game over, allowing a cadet branch to inherit from the main branch, but not the other way around, and allow inheritances between cadet branches. Naturally that will depend on inheritance laws.
Another issue are matrilinear marriages and exploitative marriages, IMHO the marriage game is an important part of the game, and marrying up should still be possible. However that might need some rules, so that a lowly count or courtier will end up in a matrilinear marriage, when they marry a Royal or Imperial heiress. However a marriage between a Royal or Imperial heiress and a duke should in most cases be an ordinary marriage, likewise a count should be able to marry a duchess.
Matrilinear marriages can be the rule for bastards and landless characters, but in other cases it is more difficult and should be far less common, maybe a kind of more than two tier difference rule.
Henry II FitzEmpress traced his right on the throne from his maternal side, still his descendants traced their rights on the throne to him. In this case it may be fitting, instead of d'Anjou or de Normandie, the dynasty became known as Plantagenêt. Ironically that was the senior line of the first house of Anjou, however by that point it was a matrilinear continuation, the last female descendant of Ingelger had married a member of the house of Châteaudun. IIRC the junior Jerusalem Angevin line remained referred to as d'Anjou.
@Miaow: patrilinear the current Grand duke of Luxembourg would be a member of the house of Bourbon-Parma, but it continues the house of Nassau-Weilburg, just like how in the Netherlands the house of Orange-Nassau is continued, both would be in game examples of matrilinear marriages.