What fun! And to be able to start from scratch and make everything exactly how you want it -you are one lucky girl!! As far as ideas, my best advice is get thee to a Home Expo Center immediately! They have tons of bathroom/dressing room ideas, all set up like in a real home! Take a digital camera and snap away (ask first, so they don't toss you out!) Or any plumbing supply showroom (they all have showrooms these days, all decorated and looking like a Martha Stewart Living magazine.) Be sure to have the countertop lowered to a comfortable seated level. That way you can primp, pamper and apply your lotions and potions while seated on a lovely tufted chair, instead of standing on a cold tile floor and hovering in front of the mirror. Have your builder install a vanity-type table instead of a cabinet - that way you have knee room under the counter. Choose your lighting wisely. (Ask for help at Home Expo/plumbing center - they have designers there for just that purpose.) Most electrical codes for bathroom design require flourescent lighting, but you can add a standard bar-type light over your makeup mirror for targeted illlumination. Also, make sure the mirror goes all the way down to the backsplash (remember, you will be seated!) to give you full viewing access.
One more idea, and this one is totally fun - go visit model homes! Or, if there is a Parade of Homes in your area, take full advantage. Check local listings for any Builders Expo or Home Design Expo that might be happening in your area. These are great places for design and decorating ideas. Best of luck!