Good choice, its so nice being able to do everything undercover without getting wet.
Do have doors either end of passageway/more than one door so that horses can get out if the other door is blocked and also helps the airflow,
windows to the outside which can be closed are great for keeping them entertained and helps improve ventilation,
wide enough passageway so that you can tie a horse up and still get past, the one at work is 12 foot,
have an area for storage of mucking out stuff etc,
rug racks on the outside of boxes for hanging stable rugs etc so they are close to hand,
an area for hanging wet rugs,
wash room is a great idea, would recommend a gas horse shower over an electric human one as they tend to freeze no matter how well insulated they are.
at work we have wooden boards which can be replaced when required with galvanized bars above, the boards are painted once a year with yacht varnish and are still going strong almost 21 years later and have stood up to some serious abuse.
make sure drainage is good to prevent wet beds and wet running out into passageway,
choose good lighting to prevent shadows in boxes, strip lights over each box are normally good.
Have great fun designing them and look forward to seeing the progress
Less excited that Gus will have to live out whilst its being done, i think i'm going to pop him over to next doors yard for a week or two whilst its being done as i can't stand him being out 24/7!