I live in an area where you build primarily for the heat of summer, not the cold of winter. I consider the siting of a barn a major, major consideration. The barn that takes advantage of prevailing winds will feel much better than will one that does not allow wind to some through. OP, I hope you can choose that and/or your original barn builder was smart about that. Of course, if you are building for winter, you would hope for an orientation that was probably perpendicular to the one I had in mind. This is true for either a center aisle or a shedrow barn.
A major rain storm here just convinced me of the need for building at least short runs off of stalls. The first 12’ will be under cover and matted. But I wasn’t sure how close from there I’d have a gate leading to a shared sacrifice area. While the sandy soil here drains very well, you probably would have an easier time and spend less money on caring for the stalls if you had runs with that “back porch” shelter for them to live in. That’s true at least during the worst of the storm when water pools on the ground.
One thing I haven’t seen mentioned yet that I’m going to try to rig up is a way to cut a hole in the outside wall of the barn high up under the eaves where the fan would go. I’d like to be able to point that bad boy into the stalls or out to the back porches. You see, after I have spent all my money on the palatial barn, I won’t have any money left for shavings, so I’d like to figure out how to keep horses outside whenever possible.
I know: I’m that kind of crazy.
In any case…
i’m a big, big fan of building for medical stuff. To me, that includes being able to take down a stall wall to create a double stall, having a way to make paddocks of graduated sizes for the horse who has a long lay-up that progresses from stall rest to a very small paddock and on up. Runs and electrical braided tape will let you do that very easily. It’s also good when the horse doesn’t have to move anywhere else just to get a slightly larger paddock.
Along these lines, I also think about having great light, a way for a vet or farrier to back his truck right up to where he’ll work on a horse (though I have never seen a vet need to back his truck down an aisle to get to the stall of a sick horse… and I’ve seen some really fubar horses getting treated).
I also like to have tie-rings in stalls. So many uses for those-- drying horse… pulling-back horse that you aren’t going to fix for some reason… two year old who needs to learn to stand tied…guests such that you need more places to tie horses being tacked up than usual…a way to keep the damn barn aisle clear because cross ties there guarantee traffic jams and do make some horses edgy.
And every farm needs a shop/junk room. It just does. Build it in.