What is Natsu hiding under the bandage on his right arm after he came back from the training?

In chapter 418, when Natsu came back to Magnolia, you can see that he had both his arms bandaged. He took off the bandage on his left arm when he went to Lucy's house. However, he never took off the one on his right arm.

Up to chapter 446, the latest chapter at the time of writing, I have not fount a point when he took off the bandage on his right arm. He wasn't bandaged prior to chapter 418, before he started the training. This leads me to suspect that he has something on his right arm which he doesn't want other people to see.

At the time of writing (chapter 446), it is not yet known, what is beneath Natsu's bandages.

The only thing we do know is that (the magic on) Natsu's arm reacts to the Spriggan 12 (or maybe just strong opponents in general). In the latest chapter (446), Natsu's arm reacts for a second time to one of the Spriggan 12, Alvarez Empire, Spriggan's personal guard. Other than that, nothing really happened to his arm.

There are many theories out there as to what it could be, but the most popular (or probable) one is that his arm is turning into that of a dragon. Up until before the second time skip, Igneel had been preventing Natsu from becoming too strong, in order to prevent Natsu from turning into a dragon, but with Igneel gone, nothing had been restraining Natsu's progress any more, so my guess would be that his arm has turned partly dragon. Other theories are that he had obtained Fairy Glitter during the time skip or that his arm is turning into that of a demon, because his etherious side is waking up. Maybe Igneel was not only trying to keep Natsu from turning into a dragon, but also from turning into a demon (aka E.N.D.).

Obviously many more theories could be thought of, but, as I mentioned at the beginning, nothing is known about his arm yet, other than the fact that the magic reacts to the Spriggan 12.

I do believe it has something to due with E.N.D., however I do not think that it's a demon mark (like the ones that Gray has when using Demon Slayer Magic) but something else. When looking at thiswe see that in the last Dragon Festival (Civil War) that a man came and interrupted the festivities and killed many a dragon and foe alike. People say that this human was Acnologia and that Zeref cursed him into becoming a dragon, it's even stated in a book about Dragon History. The thing is that we've seen all Acnologia's human form, so this statement isn't exactly true.

Igneel also heavily implies that all dragons were once humans that learned Dragon Slaying magic, and that was why they were inside of their proteges: to make sure they don't make the same mistake as they did. So the real question is, who is the human in the picture? And the answer is Natsu.

There is also a time when someone (I believe its Mard Geer or Atlas Flame) talks about E.N.D. 400 years ago and how he killed so many dragons that E.N.D himself became Dragon-like.

Now look at the photo once again, notice how Natsu's scarf is on it? All we have really heard was that Igneel gave Natsu that scarf as a good luck charm, but that was from Natsu's words himself. He obviously doesn't remember his time as E.N.D. so maybe Igneel just told him that he gave it to him.

So while I do believe that he is making sure the rest don't find out about some sort of demon marking, I doubt it's like the ones that Gray had. You know, since he still has to hide them.