Chocolate and cake are such famous friends, there is an entire collection dedicated to Chocolate Cakes on the Great British Chef's website. Aside from cocoa, the other most popular sponge flavouring is vanilla. Don't be restrained by convention though. Lots of dry flavourings can be stirred into a plain sponge mixture - see Marcello Tully's pepper sponge for inspiration, or even look to saffron and matcha tea sponges as a way to introduce both colour and flavour.
Another popular addition to a plain sponge cake is fruit. This can be baked into a cake, or served with it as a filling – either as fresh fruit or jam, or even dolloped on top, as with this flourless lemon cake with rhubarb compote.
Perhaps the most common way of introducing new flavours is through decorative techniques. This might range from flavoured or plain glace icing seen atop of the British fairy cake, to the dramatic mascarpone, cream cheese or butter cream frosting used on American cupcakes – then studded with sweets, fruit or chocolate. Other cakes might use flavoured ganache, fondant icing or even more elaborate touches like sugar flowers or crystallised petals.