I've often wished I had the horse barn at my parent's place on my property so I could.
A 12x12 stall with existing human entrance doors and outside access animals doors is perfect IMO. We built dutch doors for the horses, a pop door would be easy to install for daily use, but with the larger door an option inside or out, you have clean out options. Every stall is different, so the customization is going to different for each, but in day dreaming of a larger chicken barn someday, with breeding pens, etc, I keep coming back to a monitor roofline barn with a center aisle and stalls on each side.