Zeref is a Dragonslayer?!O_o
I think there is the possibility of him being a dragon slayer since there ain´t much magic besides dragonslayer magic in the series and Zeref can kill you just with expanding a dark aura.
Also dragonslayers have attibutes unique to their fathers, their dragons.
Zeref´s character have something unique to Acnologia, Acnologia sees humans as insects but being Zeref could have been what made Acnologia aknowledge him as a baby of sorts.
All in all, there ain´t much evidence but i truly believe that there will come a chapter where Zeref will suddently come out with the craziest dragonslaying magic and push Natsu´s potential to his limit as a dragonslayer.
Also, Zeref disappears for centuries or something, same with acnologia then, Zeref appears and leaves the Tenroujima island, Acnologia comes in to destroy it as if to wipe out any trace of Zeref´s whereabouts.
As there ain´t much logic in fairy tails structure, i can´t come with much evidence to support my claim but when Zeref does some Dragonslayer magic, refer back to this thread.