A photo of the room as it is, along with pics of the shelves as they are currently would be a good starting point for our suggestions. Unless you are uber organized, I would think there might be a way to 'tighten up' the items on the shelves now, to give you a bit of space to place things.
As for the window, you could look into some board and brackets and put a couple of shelves there. Also, look to Ikea for one of the rod and bucket fixtures or make one to also go in the window to hold smaller items like pencils, scissors, etc.
As for a design wall, go for the large piece of insulation from Home Depot, cover it with flannel and place it in front of one of your existing shelves, leaning against it. it's very lightweight and you can move it when you need to.
Ironing if it includes large pieces might have to be done outside your room. Meanwhile, one of the June Taylor ironing mats would be perfect when piecing. That's all I've had in my sewing room for years and it's great. I can place it against a wall when I need the space for cutting. Hope these ideas help you.