Lucky, lucky woman! Lots of good ideas here. Inventory and measure what you have and what you'd like to have. We've designed and built two homes; both times I used graph paper and furniture templates to the same scale for my furniture placement. Let's me know where to put doors, windows, etc.
Also consider electrical outlets at table top height for easy access.
I'm dreaming and scheming for my own sewing area (fell to the budget axe when we built :-< ) so this is very interesting to me!
One of the things I've been thinking about is a way to screen at least part of my mess from visitors - maybe face the back of my machine to the door/sitting area with a partial height wall.....plus I like to know when someone comes in (my back is to the door now and my husband thinks it's very funny to sneak up on me - yikes!). I also prefer my natural light coming from the side rather than facing a window or having my back to it.....
Track lighting!!