Hi! I'm here again just as I promised. This time I wanna make a theory about the dragons returning in FT.
First I think Acnologia don't come back in this arc because he decided to watch what happens between Alvarez and Ishgar. Acno will be in the next arc and he will be the final opponent.
And now the dragons returning part. Any of you read Christopher Paolini's Eragon book and its sequels? In the books the dragons also extinct but they hide some of their eggs before they got killed by the evil king and his minions. I think something like this happened in FT. Or maybe Acno stole the eggs from the dragons before he killed them. Anyway I think Acno knows where are the eggs.
And in the end of the Alvarez Arc someone from the Spriggan 12 or Zeref himself will tell this to FT's mages. Of course there will be three person who want to bring the dragons back to the world. But they have win the argument over it. And there our heroes set out a journey to another country to accomplish this goal.
In the road to the eggs the heroes get some knowledge about Acno and his minions. (I think he also have some.) The find the eggs but in the road back to FT Acno reobtain them and want to destroy them. In the end they must fight to defeat Acno and to bring the dragons back.
Please remember this just my theory. Thank you for reading and commenting it. And please correct it if I spoiled something.