Zveroboy1

A torrent would definitely help.

You don't need a real 360° panorama, but surely you'll need a very long picture or a collection of pictures. Just think about this: each original game section for the horizon is 2048 pixel long, you'll need 8192 pixels to cover the entire in game horizon.
 
Now, first of all your reference picture must be at least 8192 pixels long, because merging different pictures is practically impossible (not just for the different sides, but also different colors, etc.).
 
Consider that if you want a better result than the original horizon you most probably want to double the original resolution, otherwise your modded horizon won't be much better than the original one, like a 4096 pixel long file per horizon section, that's 16384 pixels to cover the entire horizon.
 
So, to make a decent horizon mod you nee a picture:
- long at least 16384 pixels
- taken at the right time of the day
- depicting a landscape that fits the game's
- without oddities such as a human face picking from a corner or any kind of foreground obstacle
 
You don't need a 360° view because you can Always stretch the original picture so that it covers the game's fictional horizon, problem is you can stretch a bit not too much, too much stretching and you'll see it by eyes there's something wrong. 
Another problem: preceived distance, you'll need a picture that does not seem too close otherwise either the horizon will be a joke or the battlefield will look like legoland.
 
Now, don't forget about the business, that 16k pixel horizon needs to be cut in detail, because if you skip a 4x4 pixel shape somewhere I guarantee you'll notice that in game, as a UFO in the sky or a hole in the ground.
Suppose you use a 20 pixels large brush to do that, go figure th amount of patience needed to pass that brush all over 16k pixels (16k/20= 800)...
Then you need to make sure the sides are merged perfectly, so you'll need the cloning tool and a lot of cut and paste to align the pictures perfectly.
Then you need to take out all the small oddities that are in the picture, again using the cloning tool at a micro level.
Then you need to adjust color brightness, contrast, saturation. Considering that these operations might ask for more corrections.
All the above while loading the game over and over again to check what it looks like.
 
After all of this, you'll need to make haze and thick haze variants, most probably by adding a mask, making sure it fits the game colors and the sky colors.