Lots of natural lighting as well as electric would be nice. If there are not enough windows, think about skylights in that lovely vaulted ceiling. I think hard floors as mentioned are good. Wood laminate also holds up well to scuffs of a rolling desk chair. But do you need to consider acousitics with accordians? Will you need some soft surfaces to absorb some sound? Maybe some large area rugs in the music and sitting areas. Draperies might also help if the music getts too loud.
Is a longarm quilting system in your future? If so, you might want to think about placement of that and not make permanent builtins like cabinets in a spot that you'll want to expand to in the future.
Some previous threads talked about building large homemade ironing tables that make ironing a length of fabric so much easier. If I wasn starting from scratch I'd want one of them for sure.