Okay, this is a lot more than ONE thing, but I'm planning a new sewing room, which I hope to move into this spring, so I have a list!
I gotta have shelving for my stash that is spaced closer together, even if we have to build it. The shelves I use now are spaced far enough apart for the file crates that hold my fabric, but the full crates are getting too heavy for me to lift. And the piece I want that is ALWAYS at the bottom of the bin. So my new system will have shelves only about 8" apart and the fabric is coming out of the crates.
Pegboard, pegboard, pegboard, can't live without it.
My new space will definitely have a design wall, but you already have that in the works. I agree with kyquiltlover above, I like cotton flannel better than felt or fleece. Cotton fabric seems to stick to it better.
I will have a pressing station that stays in place at all times.
If you have the room, a large cutting table that you can access from at least 3 sides is wonderful. (Your pressing station could even be at one end of this table.) My cutting table is made from two cheap microwave carts from Kmart with a huge old desktop mounted across the tops. The carts are both on wheels so the whole thing rolls anywhere I need to put it. We spaced the carts back to back and far enough apart to mount a set of Closetmaid style wire drawers between them. These are great because I can access the drawers from both sides of the table. I mounted peg board on the sides of both the carts. The desk top measures 36 X 60" --big enough to baste large quilts.
One last thing, a deep cup-holder that clamps to the table so I can't knock my coffee off in the floor anymore, or all over my project! Enjoy your new workspace!