I don't have a barn for my ducks yet.
I did grow up on a working horse and beef farm. I had three goats for pasture buddies, too.
If you're going to breeding and raising goats or other mammals. You'll want an oversized stall for birthing and baby care until they can be released to a safe pasture. This stall can double a hospital stall when you need to treat/isolate an individual. This stall can also serve as a weaning room when you wean babies from mama.
Goats can be susceptible to coccidiosis so it might be best to have their area separate in the structure (no sharing with other species).
You'll want an area for medicating/grooming/hoof trimming. Cross ties/head gate etc.
If you're going to show your goats consider running a hot water line and shower to an area with a drain. It's much easier to bathe in cross ties in the barn than it is to bathe on a long line in the yard by the faucet.
I'd consider adding openable dutch doors or windows in each stall. Dutch doors if you're going to have small paddocks directly off the stalls. Windows so you can open them for ventilation.
Consider a cupola or ventilation fan/ridge vent on the roof. Stabled animals need airflow to keep the air quality good.
Consider adding skylights to help with the natural lighting. Either fiberglass panels inlaid in roof or actual sky lights. Our home has bubble lights which use mirrors to intensify the light. We never turn the lights on in those rooms in the day.
I'd also consider a tack room /tool room for storing nonfood items like halters/leads/medicine etc. Devoting a tiny area for those things helps your whole structure feel like it has more room.
Please don't store hay above or in the same structure as your animals. It's a terrible fire risk and the dust/mold particles in hay really lowers the air quality. I'd keep no more in there with them than can eat in a day or two.
Electricity, obviously. If you want to use regular incandescent bulbs please use a fire cage.
You may also want to install cameras so you can watch for labor signs. I know from experience that it sucks sleeping in the barn while you're waiting for an animal to start labor or running in & out every hour to check on them.
Large single or double sliding doors on both ends so you can open them for ventilation.
Keep the center aisle free and clear. You never know when you'll need to use it to stall/bed down an animal. We've used this strategy with untamed horses so they'd have a larger area to be in at first. Then they'd get moved into a stall once they were not so scared of people.
Consider how you want to handle the waste material each species creates. You may to put in floor drains and drain lines for the ducks and the goats to help drain moisture away. If you go this route then you may need ot consider where the males will be kept and where the females will be kept. Because they release urine from different directions. Boys need drains in the middle and girls most likely need a drain towards the edges. But honestly girls tend to be more gross in their stalls than boys. Poo everywhere.
I've worked with both composting and drainage systems. There are pros & cons to both.
Finally, depending on your area you may also need to plan for a generator so your barn has power if you lose power due to weather etc. I always have to consider this because if we do lose power it's out for a week to 10 days at a time.
PS- you can always contact your local ag extension agent or Ag department at a land grant university. The professors can help you with facilities design questions.
PPS- Goats absolutely love to eat horse tail hair and human hair. I bet this goes for feathers and bunny fur too. Don't ask how i learned that they like to eat human hair.
