We built my sewing room for this sole purpose more than 20 years ago. The room has a space for my computer several sewing machines and most importantly a bathroom. I know that is something that you may not think of with a sewing room. When I am on a roll with my sewing I don't want to stop unless I absolutely must. The bathroom is a necessity.
A lot of cupboard space for fabrics and notions. I have specialty drawers in lower cabinets that have 10 pull out drawers for notions and supplies.
The only big problem is that after 20 years I have run out of space and now have to go through things and decide just what I may need to give up. I have enough fabric to out live me by x3 and every thread and now 14 sewing machines. All the machines would now be considered vintage or close to it. Someday I will purchase a new machine.
I also do not have the space for a design wall. I can put one up but I would be giving up something else. I don't want to do that so I use the floor for my design wall and that works ok.
Cabinets, cutting table, computer area and bathroom. Those are the primary for me.