Everyone keeps saying "oh that's Romeo not Zeref!"
but, what if Romeo and Zeref actually are the same person?
I thought that first, they bear a Stricking ressemblance
then, you have the fact that, not only Zeref knows Natsu, but he also seem to adore him and also ADMIRE HIM.
Romeo always dreamed of becoming a Mage, and then 7 years later he's branded by a mark.
Also, if Zeref/Romeo went into the abyss of magic, then he must've known Time Arc, and thus maybe leading him to go back in time where he would ACTUALLY BE Zeref the dark mage.
Nyway, just suppositions and theories, but I'm damn serious.