Some elements to consider:
Access: for emergency vehicles, for large delivery vehicles, for horse trailers, adequate space to turn round, space to store vehicles.
Ventilation: the better the ventilation the healthier the horses.
Drainage: sloping floors, accessible drains, safe disposal of dirty water/fluids. There might be tight local rules to protect waterways.
Light: as much as possible for working in the stalls, for vets and farriers, for dark winter evenings. Electric supply or green generation such as solar on the roof.
Muck disposal: consider the not inconsiderable amount of muck horses produce all year round. How to move it out of stalls, where it is stored, how it is disposed off.
Clean water supply: not just for horses but for laundry, for general cleaning - and importantly disposal of waste water. Consider water stewardship and collecting rainwater and grey water.
Storage: not just hay and feed stuff but endless amounts of stuff associated with horses. Dry, airy, well lit, well organized. Space for wheelbarrow, space for wet rugs…
Fencing: look at modern materials that are safer for the animals and require less maintenance.
Aspect: learn from local farm buildings about the best aspect for doors and windows for sunlight and protection from bad weather.
Consider climate change as extreme weather events are becoming more common.
Consider using recycled materials as much as possible.
Maybe obvious, but put the barn close to the main road access point so as to preserve as much graze and turnout as possible.