On one side we have hiro saying that it has a long way to go, then other side we have him naming the chapter as prelude to final battle.
Well lets see
we have spriggan 12 to deal with(40-50)
then we have zeref and Fh(8-15)
then we have gray vs end plot(5-8)
and FINALLY we have acnologia, which according to me will take long time
we might get a flashback of dragon civil war, we got like 16 chapters of zeref and mavis backstory
so lets assume we have a flashback(10-15)
acno's final battle(8-10)
And then we might get ankhserum to deal(5-10)
At last we have an epilogue of maybe 3-4 chapters
so that makes roughly 79-102 chapters left
also the story doesnt need to advance in this particular order, but this gives a general idea of how much chapter remains
which means a roughly 2 years, but given hiro's capability he can draw that many chapters in a year
Let us put all the things that haven't been explanined.
>>END
>>Acnologia and Relations
>>Dragon Civil War
>>Yakuma 18 Battle Gods.
>>Origin of Gods/Ankhselam
>>Shadow of Rogue
>>Existence about other Dragon Slayers along with Acnologia from 400 years ago.
>>Location of exceeds that are sent to catch Dragon Slayers along with Happy, Carla, Lector, Frosch. More than a few has been sent to the earthland along with Happy & Carla. In total 100 exceeds have been sent by Queen Shagotte.
>>Layla Heartfilia
>>Celestial Spirit King and Mard Geer.
>>That Big Map. What we have seen till now is only the West of Fairy Tail.
>>That cat of Zeref which appeared in GMG arc.
>>Gray's Demonification?. and why?.
>>Creation of Nirvana 400 years ago and 'Nirvit' race.
>>I guess there are some more I want to say.........but we will see...