Oh, I forgot to mention something... you said you were looking for a filling for your cake.
I tried this the other day and it was very good. Take a box of instant vanilla pudding and mix it with sour cream and milk. Then blend fresh raspberries in the mixture and use it to "fill" the layers between the cake. I spread a very thin layer of raspberry preserves before the filling so it would have something to stick to.
It goes good with butter cream (IMO) as it is not too sweet. I also think this would be good with blueberries or blackberries.
