Before you plan your closet, decide what you want and need to store (wants and needs are two different things!). When I was planning two walk-in closets, it was suggested that my hanging space height would be based on my height-a size 2 petite would need a different height than a size 10 long. ( I am not 2 petite!) Measure the height of your folded sweaters- how many do you want on a shelf so how many shelves of a certain size do you need? Do you hang your jeans or fold them? I used the wire systems from ClosetMaid since I lived in Florida and air circulation is important in that climate. The advantage of the wire system is the flexibility to change if you want.
In my closet, long dresses had their own space, while my work clothes were mostly shirts and slacks, so I had two racks. My DH needed shelves for work boots, separate hanging space for golf shirts and shorts, hung at a different height than my shirts and shirts since he is taller so his clothes are longer.
My best friend is masking tape. Use it to mark out your ideal storage areas in your new closet before you decide on a system or buy anything. How high can you comfortably reach? Do you need a shelf for special but seldom used clothing like ski stuff? How wide do you want your shelves.