So two predictions I've been theorizing are now confirmed.
#1 - Natsu's flames have reached an increased temperature to such a degree that they can now effect other properties in a bigger area of effect. For example the stone that made up the Grand Magic Games arena was encased in flames and started to melt and deteriorate. And now with sand, it simply evaporated like water, both of which took merely seconds. I don't have any idea of what training methods he did, or if Gildarts showed him anything but maybe it's something to do with Igneel gone as he was a potential limiter of sorts.
#2 - Natsu's arm has now reacted to both Brandish and Ajeel. I'm beginning to wonder if it's either because of how high their magic power is (Which seems the most logical case) or if it's something that Zeref has done/given to all the Spriggan 12 members. If that's the case, then any of the 12 that Natsu has to take on seriously could perhaps reveal the secret of Natsu's bandaged arm.
I'm not really bothered about theory number one as it's not really a theory per se, and it most likely won't be touched upon. However, at this point in time, there are 2 logical and sensible answers for theory #2 and what it what it could be.
#1 - E.N.D - I don't really need to explain anything about this, given we all know it's confirmed now. The problem is, the only way Zeref could awaken E.N.D is if he did something to Natsu's body when he brought him back to life. That would mean that the Book of E.N.D is merely an activation of sorts that corresponds with whatever Zeref did to Natsu's body which would then cause E.N.D to reawaken.
#2 - Natsu's upcoming Dragonize Transformation
Before anyone says that it's only a movie thing, hear me out. We know that if a Dragon Slayer uses their magic excessively, they can end up turning into a dragon themselves. It's not exactly stated to what extent, but it still poses a problem for them. The solution to stop that however, was revealed by Igneel after he emerged from within Natsu's bodies, and stated that they had been staying dormant within their bodies to act as antibodies to halt the process of Dragonification.
With Igneel now gone though, there's nothing stopping Natsu from turning into a Dragon. Logically, I can assume that it would take some time for the effects of the antibodies to wear off, but we've had a year time-skip so maybe it's worn off. This evidence for this theory is concrete and is backed up by Achnologia being able to return to a human despite excessively using his own Dragon Slayer Magic in the past.
TL;DR
Either way, Natsu is bound to receive a new power-up in some form in the future, one that's been fleshed out by key bits of information stringed across hundreds of chapters. Something that's not been bound to the "Power of Friendship" stereotype that A LOT of people like to use, even though they sorely misunderstand it. I'm looking forward to this arc a great deal. The Tartarus arc is the best so far but we ended up getting blue-balled at the end of it.