I have 2 small bedrooms dedicated to sewing. I have what I need, not near what I want. It is cozy and serves my purpose in the quilting and sewing world.
In my dream world I have a 4x the space. Room to walk around a wide cutting table (Mine now is 48 x 60, but attached to the wall at 48" end. I wanted 5'x5' when we redid it last weekend, but I gave in on that foot in order to stand in front of the ironing board!) Room to have my extra large ironing board away from the wall and plenty of room to walk around it. Room for storing bolts of batting properly and easily managed. An area set up for embroidery, one for serging, one for piecing, one for quilting. A large design wall. An area to display what I have finished. An area for designing and displaying books. An area to put all my fabric and notion cabinets together. Separate area for cabinets of yarn and other crafty things. Proper lighting and windows throughout. Enough outlets without having to use extension cord. Wall space for a tv that can be seen from all sewing chair areas. Place for a microwave, coffee pot, a red retro popcorn machine, and a mini fridge. A bathroom close by. Room for my doggies to lay around and visit often.
I would do this on the lower level. I would think that the weight of it all probably isn't best suited for a loft or above the furnace or a spare bedroom, as I also want a frame someday. I wouldn't want any overnight guests to hear the humming of the machines, as there is midnight sewing.
My sewing is out of eyesight for anyone coming over. I prefer that. I don't feel the need to put anything away when I am in the middle of something and I would certainly forget what I was doing if I had to do that. If you had a large empty basement, think of the possibilities!