I'm more interested in Zeref and his relationship with Mavis and FT (namely Natsu) than I am with Acnologia. As it stands, Acnologia is a villain with immense power which has had its past revealed. We already know Acnologia was a human at some point and from killing many dragons has become a dragon himself, something that was hinted to be in one way or another related or help by Zeref. So acnologia is simply one to beat, no big mistery there. At least for me. Granted we still don't know what happened to the rest of the dragons but thats a mistery relating to the dragons themselves, not acnologia. He will indeed become human at some point and be beaten by FT. Its something I also foresee happening.
As for Zeref and Mavis, I'm very hooked in this relationship. They know eachother and Mavis is obviously not a human mage or at least not a normal one. She died yet she is still alive and can still use magic to some extent. Zeref and herself are acquainted which meant that they might have fought at some time in the past. Zeref knows of natsu and was expecting natsu to somehow beat him. Igneel might have known about this relationship or about something we still don't know because he gave to Natsu a scarf that, despite seaming very simple, has been very important in the one time we saw Zeref and Natsu interact. So much so that the scarf needed to be healed and thus was related to Natsu's health.
One thing I find interesting as well is that Ignell, Grandine and Metalicana supposedly know eachother or were great players in terms of the Dragon power side. Yet all of their 3 dragon slayer children are in FT. Which is interesting. Perhaps because of the Lumen Histoire? Perhaps Mavis knows more about the Dragons? Its weird no one asked her anything about it btw.
The plot is thickening a bit with many unanswered questions that I just hope Hiro can manage better than Kishi did when he ravaged the plot of naruto with unanswered questions only to fail epically at explaining them... Tobi=Obito... **** me!