I am envious and still happy for you.
Lots of electric outlets. You might even want some in the floor and covered til in use.
I personally don't like skylights. I love Solar Tubes. They let in more light without letting in the cold or heat.
Adjustable track lighting so you can rearrange you studio as desired. Just use the natural light bulbs.
A large cutting table, accessible from all 4 sides! A friend used 2-4 Ikea cabinets as bases that are used for storage.
A place for a big board on your ironing surface. If you don't want it up all the time then a place for storage when not in use ... could be in the walk-in closet.
Curtains??? Maybe consider blinds or plantation shutters. Easier to dust and keep clean. Also can be opened for light or ventilation as needed.
Consider your own personal preferences. For instance, I cannot feel comfortable with my back to the door [I read mysteries, can you tell, and have some cop friends] or facing a wall. I could maybe face a window, just not with my back to the doorway.
A large door space into the studio if possible. Pocket or outer sliding barn style doors. Even screens could be used to separate it from the rest of the home if you want. I took off my French doors and bought a used sccreen to place across the doorway when entertaining other than quilting/sewing friends.
Plan, plan, plan ... keeping in mind how you function best.
Maybe even a little beverage area. And room for any friends who might sew/quilt and visit.
Enjoy the journey. Maybe even keep a journal of the process including pictures.
ali