Author's Note: I just joined this site so I'm still learning how to use it, let me now if anything appears wrong. This is not going to be a story, but rather a few theories on the world of Fairy Tail.
Disclaimer: I do not own Fairy Tail, any of the characters, or the plot. Should one of my guesses turn out to be true the credit all goes to Hiro Mashima, the creator, I am just a humble fan.
Theory 1: 7 Dragon Slayers vs. Acnilogia.
I believe that to defeat Acnilogia will take 7 dragon slayers working together. But they will have to be either 1st or 3rd generation, true dragon slayers. So that eliminates both Cobra and, sadly, Laxus. So far we know of at least 5, possibly 6, of the dragon slayers. The first 5 will be Natsu, Gajeel, Sting, Rouge, and Wendy. I, personally, believe that the other two dragon slayers will have to be a water dragon slayer and an earth dragon slayer. Now here's my reasoning: Most of the slayers have an opposite. The first pair is obvious, Sting and Rouge. One is light and the other is dark. But there's more to it than that, Sting is blond, whereas Rouge is a raven. Sting is more of a rush in without thinking kind of guy, but Rouge tends to plan things out first. Even their Exeeds show their opposing traits. Frosh, like Rouge, is more quiet and likely to agree but will speak out if strongly opinionated. But Lector, and Sting, is more likely to tell you flat out what he thinks. The second pair is just as obvious if you stop to think about it. Wendy's opposite is… Acnilogia himself. Hang with me for a moment, Wendy stands for light, life, air. She represents not only the clear tranguility of an open blue sky, but also the strong unwavering torrent of a storm. She is all about possibilities. Acnologia is Death incarnate. He is the darkness the surrounds you as you take your last breath. He is nothing and everything. He is the absence of life. Even in their physical appearances they contrast. Wendy is a small, pure little girl, lithe and almost pixie like. Acnologia is big, scary, rugged, and kind of buff, like a troll. The two dragon slayers who don't have matches are Natsu and Gajeel. Gajeel is Iron, metal, a man made creation full of complexity. His opposite is earth a simple, natural thing that can be found anywhere anytime. We haven't yet met Gajeel's opposing slayer. Natsu is fire and his opposite element is easy to guess. Water: calm, cool, full of purpose, and loyal. Whereas fire is raging and harsh and full of passion. Unlike Gajeel paired dragon slayer, I believe we've met Natsu's: Juvia. Which bring me to theory 2.