Perhaps you don't want a themed cake, but something a little simpler, then look at a drip cake.
Cover in an array of chocolate, sugar sweets, a colour theme, there are so many choices.
Make your drip cake a little taller and have 4 layers of sponge rather than 3, so you could even have different layers of sponge within - two chocolate and two vanilla, or perhaps white chocolate and two coffee, the choice is yours.
A price guide for buttercream iced cakes:
6 inch round - £55
8 inch round - £75
10 inch round £105