This layout is perfect for a small boarding facility. I love the tucked away bathroom under the stairs and the space for tack lockers upstairs. I imagine that would be an excellent place to hide those blankets until the colder months roll around again.
Two options (after making the sizes a bit smaller to fit a mini: Option one - take out one of the stalls and turn that area into covered area. Option two - take out the tack room and make the second stall a bit smaller for tack/storage.