My favorite cream filling is the cheesecake mousse filling inside a Costco cake. I was trying to recreate that a while back, and I found a recipe that said it was identical. It wasn't. The biggest problem that I had with it was that it tended to soak into the cake itself, so you did not get that distinction between cake and filling. In addition, I cannot use it in cakes for sale because it contains ingredients that are not allowed under the Arizona cottage food law (which is probably just as well now that the temperature is above 110 again)!
As I was searching for recipes for shelf-stable (not requiring refrigeration) fillings, I found one that I am currently loving. My favorite part - when I cut into the cake there is a distinction between the cake and filling.