Mantles can be as decorative as you want with all sorts of crown, dentels, and 1/4 rounds OR you can get a hefty peice of rough cedar and some beams and make a rustic funtional mantle. What is the decor of the room where the fp will be?
Go to www.google, a SE and enter fireplace mantles. There is one site located in SCal that must have 50 or 60 pictures of mantles that you could get an idea from and even copy.
Go to some FP speciality stores and get some ideas there. A custome made mantle is so easy to build as its can be hollow and just full of all sorts of moulding material.