Ok sooo... My cousin and I were talking Fairy Tail and she gave me her theory on Natsu's bandage. *SPOILERS*
So every one knows that after the Tartaros Arc FT separates. Over that one year, Natsu has a bandage that he never takes off. Once FT is reunited, everyone asks what does he have under that bandage. He says that is his "secret weapon". Obviously he has not reveled it because it holds tons of great magic power. During the Tartaros Arc, Acnologia gets his arm torn off when fighting Igneel. Do you get where I'm going with this? Natsu has Acnologia's arm. According to what I read, Acnologia is actually a human. So that's why you can't tell that Natsu has his arm. And she said that in a manga chapter, there was a human and he said something "Where's that fire dragon?" Or "He has my arm." Honestly idk cause I'm not caught up with the manga yet. She just told me cause she loves telling me spoilers. Soo yeah... She thinks under that bandage is Acnologia's arm or power/strength. Maybe that bandage restricts some of the magic energy so he won't use all of its power. Idk but what do you guys think? I think she maybe right.