Ski, Patrincia uses vanilla mousseline all the time, and loves it. She just adds vanilla extract to taste, no liqueur or anything.
Another lovely pairing with the marble would be Hector’s suggestion of the caramel silk meringue buttercream- more time-consuming than the mousseline, but sort of like frosting a cake with buttery flan, yum! It is one of my all-time favorite buttercreams. If everyone concerned likes coffee, that’s another nice pairing, either the coffee neoclassic or the coffee-caramel SMBC.
Do you have any requirements about the color of the frosting? Vanilla mousseline will be off-white, caramel SMBC will be light tan/beige and the coffee-caramel darker tan. All would be lovely and tasty with chocolate decorations, like a band or curls.
Personally, I’m not a big fan of strawberry/raspberry buttercreams, I prefer the whipped cream-based cloud creams instead because they let the fresh flavor of the fruit shine through. Cloud creams should be paired with a lighter cake like genoise- I like genoise au chocolate for pairing with fruit because the chocolate isn’t too intense. But that is just my personal preference, many people like the berry buttercreams. And they are the only choice if refrigeration isn’t available.
Can’t wait to see the finished cake!