Hey Antonisk,
Welcome to the forum! Let's jump right in to it:
There are a LOT of ways to make Batman capes. A lot a lot. They range from inexpensive and easy to insanely complex and expensive... it all just depends on how "accurate" you want to be. The nice thing about Arkham Knight is that it's a video game, so it's open to interpretation as to what material you want to use. I'd suggest a pleather, since it'll be more cost-effective and will give you the dark shadowy look that so many Batman costumers prefer. If budget isn't a constraint, then textured latex is how they did it in almost all the movies, although the Christian Bale Batman went with a darker crushed velvet look - which was great at absorbing light, and was a lighter material, so it was more 'flowy,' although to me it always looked furry. (and I was a Batman at the Six Flags Batman Begins stunt show for 3 years, so I was near those things every day).
Then there's the matter of patterns. There should be a few different patterns readily available online for free - I'll see if I can pull up any of my old ones. The trick with the Arkham suits is that most of them had a yolk - a heavier neck draping, or the part that's in front of his shoulders when he's standing still. If I remember right though, Arkham Knight DIDN'T have that, and it actually just hung off the shoulders to the back... so all the better! One thing that is different about the Arkham Knight cape though is that the back panels were split in to two sections - the uppers and the lowers. Which really should just be layering panels to your liking, but maybe I'm wrong? In hindsight, I wish I had taken better photos when they had the Arkham Knight art exhibit down at SDCC, but these are the photos that best showcase what I'm talking about:
View attachment 986674 View attachment 986675 View attachment 986676 View attachment 986677
You can see that the panels that split the back are highlighted with white paint in the earlier images, with a completely different piece on the upper back. Since the cape mostly drapes behind, you really shouldn't need to worry about "wingspan" too much, unless you plan on doing some elaborate poses.
As with all things, I'd pattern it out on paper one piece at a time, and once the paper looks to be about the right size, I'd transfer it to a cheap fabric like muslin to just quickly whip things up. If everything's looking good, then it's time to pick your final material and hop to sewing!
I hope any of this has helped, but I'm sure a lot of others around here will be able to help you out too, and hopefully you'll get your cape licked in no time!
-M