If you look at all the unanswered questions of FT's plot, they are all gathered in this arc or have been answered:
-Where did the dragons go: check
-What was Fairy Heart: check
-Zeref and Natsu relationship: check
-Mavis and Zeref relationship: check
-Acnologia's relationship with Zeref: more or less check but undergoing none the less
-Gray's dad: check
-Erza's Mother: undergoing
-Natsu vs Zeref: undergoing
-Acnologia vs FT: undergoing
And the only things pending are really now in play in the current arc (Acnologia, Natsu, Zeref, Mavis, Erza, FT as a whole, Gray) so like Cali says, it has all the makings of a final arc. However, I don't think FT will end just yet. Sure, it has 500 chapters now and most series have been ending at around 700 or so but I feel it will go on a bit more or at least into a new generation.
For example, Naruto's main plot was Naruto's progression into Hokage. The story was not about ninjas but about Naruto. One single character. One Piece is not about the Strawhat pirates but rather the search for the One Piece, the biggest threasure of the world. This means what once that main plot ends, the manga has nowhere to go and ends.
When it comes to fairy tail, the manga is about the Guild Fairy Tail. Its not necessarily about Natsu, Lucy and the others. The first chapters showed clearly that the focus was fairy tail even if we have main characters and fairy tail itself has been relegated to a secondary role. I would not be surprised if the manga progressed after the whole Natsu thing with new characters of Fairy Tail. And it wouldn't be a bad approach to use the existing world and lore but with fresh new characters once you solved and fixed all the plots of the current ones.