Chapter 400 revealed the most shocking revelation I've ever seen by showing that Igneel was hiding in Natsu for the past 14 years. Since we have 7 slayers this gave me 3 theories related to all 3 generations. ~
Theory #1, 1st Gen Slayers: The 1st generation Slayers were taught by actual Dragons and all disappeared at the same time. As Igneel revealed they hid themselves in their Slayers, the question is why? And how did Grandeeney speak to Igneel if that's the case?
I feel the Dragons were somehow trying to bring the Dragon Slayers together, hence why Natsu, Gajeel and Wendy all joined Fairy Tail. And that may mean there's still many more Dragon Slayers as a result we haven't met yet. This also explains why the Dragon Force for each slayer looks like their respective Dragon.
Theory #2, 2nd Gen: Since Laxus is KOd and Cobra can't be seen its harder to tell, but with the reveal Igneel gave us I think its possible those Dragon Slayer Lacrima might be in a similar situation to the Exceed in Edolas, they were made into Lacrima.
Its possible that something turned some dragons into lacrima and when they are put into someone's body they gain that dragon's power. This can explain how Laxus and Cobra's elements were specifically Lightning and Poison.
Theory #3, 3rd Gen: Sting and Rogue reacted to Acnologia appearing, but they killed their dragons already... or did they? If Dragons are able to use telepathy and the slayers could barely hurt the 7 weaker dragons that the Eclipse Gate brought then something tells me they didn't kill them.
Its possibly Weisslogia and Skiadrum used something like telepathy when they were asleep to make them think they did and then they went inside Sting and Rogue to keep up the illusion. I'm not sure if they are secretly alive or truly KIAd, but if they do show up I don't see any better way to explain how they're alive.
~ So, these are my theories that I formed after Ch 400 revealed Igneel was inside Natsu all along.
So what do you think about these theories? Do they seem possible, think there's more evidence that I missed or something that makes these theories less likely, or maybe you have different theories entirely? Feel free to give me your ideas on the subject.