Stand Mixer! A kitchen isn't complete without a stand mixer. It doesn't even have to look like a KitchenAid: it can be something more retro, to fit with the other retro-styled appliances. Though of course a KitchenAid-inspired look would be good, too.

Optionally, the stand mixer could be a requirement for baking certain cakes/desserts. Not the birthday cake, because I've always imagined that to be a boxed cake mix, but some of the fancier desserts could make use of the stand mixer.