Personally, I think that whatever's going on with Natsu and Zeref, it doesn't have to involve time-travel. Natsu and Gazille have their ages listed as "unknown" in their profiles (which is weird, because Wendy was stated to be 12), and during the Luxus arc, the two of them were considered to be over 80 by Fried's barrier. They must age like normal people (seeing as Natsu's been roughly the same age as Gray, Erza, etc. during flashbacks and whatnot), but they're still much older than they look. Maybe they were originally from the time where Zeref was at the height of his power when they were little kids, and then they were frozen in time somehow? That doesn't explain why Zeref thinks that Natsu can defeat him, though...

He could, I suppose - he's made so many self-sacrifices over the course of the series that one of them might stick eventually - but I'd think it would kind of make all the drama involving him and Erza pointless if they never got closure on their relationship. That's just me, though; it could still happen, regardless of what I think.

That's okay; I always like hearing speculation about what could happen.