Most of the time, brides don't want custom designed cakes, as they feel that they cannot picture how it would look or if it will turn out good. They rather choose something that they have seen.
My advise would be if you are already paying for the cake, might as well make it yours! As long as you get the colour combination right, the cake would look perfect and on top of it, it is a design that will match your theme and something you will say, its mine and no one else has it.
Perhaps, try getting inspirations from websites or pictures that will aspires you and get bits that you like and match them. This will definitely help with the designing process. Even if you really like a cake design, choose it and then change some bits of the cake. Fro colours m colours to choice of ribbon types to type of flowers will make a huge impact on your cake and make it yours. These little things are just to cater and theme it with your wedding theme. In actual sense, you still will get the design that you wanted but with a little tweak in the, figurings or what not will make the difference.
Here's a little inspiration board that I've picked out from the website Style Me Pretty. They are absolutely gorgeous and inspiring when it comes to weddings.