I would add another wash stall at the opposite end of the barn and remove one box stall (there is always a line at our barn, and we have two wash stalls and 24 horses!)
Individual tack rooms are a great idea. I wish I had a dollar for every item of mine that's been "borrowed" over the years, despite my having my name stamped and stenciled on every equine item I own!
I have always been suspicious of overhead hay lofts ever since a fire started in one at the main barn at Rockridge Farm here in Atlanta many years ago. 34 dead horses. No ignition source - just improperly dried hay. And concrete block construction won't solve the problem because there's plenty of fuel from the hay itself. I think that hay and shavings or straw should always be stored in a separate building some distance from the barn.
One solution is the "Vermont barn" - built into the side of the hill so that you can wheel the hay into the upper story from a separate storage building on the uphill side. If the barn were built with a very sturdy frame, you could even tow the hay in behind a small tractor and toss it down.
Fire is always a fear, but day to day ease in a pretty rainy climate is a factor in having at least a working supply of hay in the main barn. A compromise would be to bring a few days' worth at a time in from another storage barn. I'd like to have the wall around the hay be a concrete firewall.
Another reason why the outside exits from each stall are vital.
Also good to know... I had another outside, but it can only be used half the year around here.