My initial assumption the first time I saw Mavis was that she was, literally, a 'fairy'. She was the spirit of Fairy Tail who would whisk away our heroes from their doom and protect them with some sort of magic that transports them to a different dimension (much like Anima). Interestingly enough, Chapter 255 proved to be a little similar to most of our original theories, which both satisfied and disappointed us (how anti-climatic can that chapter get, anyway?).
Now here's the thing. I've seen how Hiro-sensei enjoys doing the most unexpected twists, and now it gets to me that he's coming up with yet another surprise. The question is: just what is it? In my opinion, I think the seven-year-older Fairies played too little of a part (just one chapter, for crying out loud!) to just suddenly fall back to Natsu and cos shadows yet again. So! I get the impression that they will be playing a more major role for the chapters to follow. In fact, they might as well be the main characters of the next few updates. It could be that because of the sudden change in the environment, most of the main protagonists' magical powers dissolved. They could have a hard time adjusting to their new surroundings, so its up to the new FT to get them back on their feet.
While Natsu's company adapt to their new environment, Macao's team dukes it out against Tartaros. However, just as Fairy Tail seems to be winning against Tartaros, they draw out their ace, which turned out to be Zeref (I tried Acnologia, though I don't see that dragon teaming up with anyone, anyway), and FT's brought back on their knees once again. As they recover, Mavis secretly channels her powers to them, allowing them to fight on par with Tartaros.
The last battle obviously goes down to Natsu and Zeref. Amidst their heated fight (or as Juvia Lockser puts it: heated grappling), the dragons return. Igneel aids Natsu, as Acnologia with Zeref.
I got too carried away. My penchant for writing fan-fics is kicking again. Anyway, I'm curious as to how the rest of you view the next arc...