Assuming you are referring to the statement, "I am aware assassination is in the gang leader class, but just something to think about," I believe @GeneralMayonaise was referring to assassination ability that comes as a marriage benefit in my main post of this thread under marrying into the gang social class.
That would be correct. I was not referring to a current in-game mechanic. I will do my best to be clearer in future posts.
I kind of like the idea of keeping specific benefits for different classes, but that doesn't mean we can't do as you are suggesting! Let's take your idea and tweak it a little.
What if you have mostly nobility in your family because you do want to conquer the world through politics, BUT you also have a single gang relation in your family that you use for the nefarious work, such as assassinating someone vehemently opposed to a policy or law or a warmonger in your faction etc.. Enough noble marriages could easily counterbalance the negative opinion caused by marrying a single gang family into yours. This adds power to the combination of class marriages to reap some of the benefits from each. Of course, once the assassination plan is on the table, like any task/quest in Bannerlord, you should have the option of sending a companion to complete it "with a %chance success rate depending on a specified skill of the assassin, and either level or specified skill of the mark(s)."
It's your thread and original idea, so I will defer to you. Besides, your system is better than what I could have thought up. I keep forgetting about mixing family lines through the generations could be a factor. +1 on your reply!