I don't have any pics of hay wagons, but I have a few tips for useful features.
The three hay wagons my father had were built by himself and his father in the year 1958. We used these wagons until 1995 when they were sold at auction. I don't recall the auction price, but I do know it was quite high, especially for 40-year old wagons.
Our wagons were built on Electric brand running gear (Electric Wheel Company, Quincy, ILL), which were always my favorite for ease of maintenance and ease of use while hitching and unhitching - versus Gehl or John Deer running gear which I disliked.
The two reason why these wagons (and all my father's equipment) lasted for 40 years is because 1. No exposure to the weather -- we parked the wagons in the barn at the end of every day and never left them in the field. 2. Regular maintenance -- my father would meticulously service each piece of equipment before using it. Lubrication, bolt tightening, parts replaced, etc.
Now, on to features I always liked about these wagons:
As a farm boy, I often helped other farmers with their haying or we would occasionally lend out/borrow other farmers' wagons. So, I got to work with many different types and designs of hay wagons. I always liked my father's wagons best.
1. Made of Wood. I prefer wooden hay wagons.
2. Rough-cut lumber. Our wagons were made of rough, unfinished boards that did get smoother over the years. This prevented foot slippage or falling down while loading or unloading hay bales.
3. The front opened. Our wagons opened in the front with the front panel mounted with barn door hinges. The front panel opened downward towards the wagon tongue and was the perfect height to be supported by 2 hay bales on their sides on each side of the wagon tongue. When closed, we secured the front panel with a chain and a common chain tightener.
With the front panel opened and resting on 4 hay bales, we could then unload the wagon. No removal of bales was required - just open the panel and get to work. The open panel was now the elevated platform to stand on to reach the hay elevator. It worked perfectly. The only issue was falling off the front of the wagon if you weren't careful.
I worked on a lot of other wagons that did NOT have a front-opening door and I hated them. With a front-opening door, the hay wagons were useful for other applications, such as transporting or storing other items.
4. Side-openings. Our wagons had side openings that were in the center section (our wagons had 5 vertical posts). These side openings were closed with lumber that was a few inches longer than the gap. Ours had hook-and-eye closures with small pieces of sheet steel to keep the door in place when opened. The side openings made it easy for a man to get in and out of the wagon and made it easy to kick out the loose hay chaff.
5. Correct height. Our wagons were just short enough so that we could park inside the barns. In fact, we used to drive the entire tractor and wagon INSIDE the barns and drive right up to the hay elevator. I never got a sunburn unloading hay on my father's farm.
Here is a tip for ANY hay wagon or light-weight wagon: When you want to park them inside barn and they are empty -- unhook from the tractor and back them inside the barn using only human power. So long as it's not uphill, it makes the task quicker, easier, and prevents damage to the barn or the equipment.