User blog comment:Bunnyllove/Have Natsu and Zeref meet before?/@comment-5343732-20131111110527/@comment-24066070-20131111123136

Natsu, Gajeel and Wendy being from a different time is a possibility because of that rune barrier, but I'm afraid that if the author uses another time travel/loop or alternative timeline he will just mess up again.

I doubt Natsu would be Zeref's son. Before you know it Zeref will go "Natsu, I'm your father! Search your feelings you know it to be true!" and that would lead to even more Nakama & Love speeches.

The best explanation for Natsu, Wendy and Gajeel becoming Dragon Slayers would be that they are descendants of some Dragon Slayers who fought in the Dragon War 400 years ago. That would explain their compatability with Dragon Slayer magic and why they are stronger/ more talented than any other Dragon Slayers.