dragnoots:

Note that Cana and Gray got tattoos/marks on their right forearms when they acquired new powers (Fairy Glitter and Ice Demon Slaying Magic respectively). And after the timeskip, Natsu had his right forearm bandaged and, according to a certain colored cover (I am unsure which) under the bandage is a pretty chill tattoo similar to Cana and Gray’s. 

Did Natsu acquire new magic sometime along the one-year timeskip?

I say it’s possible. It could explain why he and Happy chose to leave Lucy behind in the dark (and the whole Fairy Tail too, basically), why Natsu attained extreme power-ups and much more magical stamina. It’s also possible that his powers as E.N.D. are showing up or has already shown up.

Any more theories on this?

Igneel could of left him some of his power as well! Like how Gray’s dad did!