What flavours compliment each other in baking?

I'm looking to make a chocolate cake with a flavouring to it. I was thinking chocolate and coffee... I'm not sure how that would taste so I thought I'd come and ask some real bakers what they think :p I was also thinking if two different icings(covering the cake in one icing as an inner layer, then the outer layer with a different icing). Is there a way to lighten chocolate icing, or maybe I could add a flavouring? Whatever hints/tips/tricks you guys could give me are greatly appreciated :) Thanks