After reading Chapter 410 of Fairy Tail, which was well executed with Mard Geer's incredible strength. Natsu and Gray are "dead" for now. But how will they come back you might ask? Well to be honest I have multiple theories, but this one seems more probable than the rest. So here we go!
Well for starters, I can honestly tell you there may be a HINT of Nakama Power in the next chapter because it's called Quid Pro Quo (Manga Stream Translation) which means: a favor or advantage granted or expected in return for something. Which I believe is going to Lumen Historie. Basically Lumen Historie will help Natsu and Gray become stronger and beat Mard Geer the King of the Underworld, hence the "Magna Carta" that limits his power or gets rid of him entirely in part 6.
I'd also like to add that this is how Erza beat Kyouka in her battle, its Lumen Historie not "because she's Erza" After all the creator is implying that Lumen Historie is going to give the Fairy Tail mages an advantage in their battles against the remaining members of Tartarus.
Also I think the rest of Part 6 will be the 20 minutes before FACE activated in Chapter 404 and Part 7 will continue with post-FACE activating. E.N.D. will be resurrected and the final villain of the arc will finally show up.
Well I guess that's all for this theory, I hope you enjoyed it and as usual if I got something wrong please feel free to share. If you have another theory please share that too, I would love I good debate.