While the flavour of your cake is limited only by your imagination, there are a number of factors you should consider before making your choice.
Do your guests have any allergies or food requirements? How many people are you feeding? Generally, the more people you are feeding with a single cake, the better it will be to choose a simple flavour. If you do want a specific flavour which some guests may not like or be allergic to, such as peanut butter, consider a tiered cake and choose a simple flavour for the other tier.
what icing will be covering your cake? If you want a fondant cake, you have to use a higher density cake, such as a mud cake. A low density cake, such as a light vanilla sponge will collapse under the weight of the fondant icing. Likewise you should consider the flavour of your icing. While the batter and icing do not have to be the same flavour, they should be complimentary. Also remember that fondant cakes will have a ganache layer between the fondant and cake.
How much are you wanting to spend? Specialised and one-off flavours will be more expensive than the classic chocolate or vanilla. Some customers also request flavours using alternative sweeteners such as coconut sugar. Cake makers rarely have these alternatives in stock, so buying it especially for your order will increase the cost.
Most cake flavourings will be made from essences, although this varies between bakers. If you would prefer the real ingredients, like using orange zest instead of orange essence, make sure you discuss this with your baker before ordering.
If you are serving thin slices of cake, such as for a wedding, you should avoid cakes with chunks of fruit or nut. While you might have your heart set on a walnut cake for you wedding, slicing this cake in thin serves will result in a pile of crumbs rather than a clean slice. So you can either choose to account for each slice to be cut in larger pieces, or to choose a flavour without chunks.
Ombre and Rainbow cakes
If you want an ombre or a rainbow cake, you should stick to a vanilla, white or butter cake to ensure that the colours of the cake stay vibrant. Adding flavours can make the colours murky, which will diminish the visual effect of these style cakes.