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.