Many cakes that I make have only the buttercream in between the layers. For special occasions or for a change of pace it is nice to add a filling. There are unlimited flavor options for filling a cake. Once you choose the flavor that you think will compliment the cake itself, it's time to build the cake. The danger in a filling is that it can cause the frosting to bulge on the finished cake. This makes the layers very obvious and the cake looks frumpy. There is a great solution! Dam the filling.
Take some of your frosting and mix it with some more powdered sugar until it is thick enough to form by hand but still thin enough to pipe through a large round tip or coupler.
buttercream between the layers as well.
truffle filling that is easier to see.
I have used this technique with jam and mousse fillings with beautiful results. I make one change with jam fillings. They tend to seep into the cake and make it soggy. I spread a thin layer of buttercream on the cake, then pipe the dam and fill. Spread a thin layer of buttercream on the top layer and flip it onto the filled layer. The buttercream creates a moisture barrier to protect the cake.
Have fun with this one. The sky is the limit on flavor possibilities!