I don't know if anyone's said anything about these things; not that they're ground-breaking or anything. But, while rereading the Battle of Fairy Tail arc, I noticed a couple of things.
1) Grandeeny speaks to Igneel, standard knowledge. However, on the page before she speaks to him, Natsu falls asleep (again), meaning that Natsu is not conscious of Grandeeny's presence. Which clears up a little confusion as to how Grandeeny was speaking to Igneel inside of Natsu's body and Natsu not knowing that there were two dragons talking inside his body.
I also think that this can be related to when Grandeeny spoke to Porlyusica. Porlyusica described it as Grandeeny speaking into her mind, if I recall correctly. So I guess Grandeeny did the same to Igneel? Or maybe even Natsu? Since it isn't exactly described as to how "inside" the dragon's were, it could be that they used magic to fuse with their dragon-slayer, so that they both experience the same thing? Or perhaps they were separate entities, and the dragon-slayers were just carrying around their dragon parent. But if the two shared the same experience, it would make sense for Igneel to get angry at Grandeeny for contacting him, since she might wake Natsu and alert him to Igneel's presence.
2) Igneel says that he will see Grandeeny at the Dragon King Festival, which leads me to believe that maybe the dragons inside the dragon-slayers somehow spoke to each other amidst all the chaos of the Eclipse project? Maybe to kinda just talk about Zeref's approaching thing??? Iunno, I think it's interesting that Mashima mentioned something like that so early.
3) Natsu and Gajeel are both unable to leave Freed's Justu-Shiki on the guild hall, which only forbids people older than 80 or stone statues from leaving. Obviously, neither Natsu or Gajeel are actually stone statues or over 80 years old. So why couldn't they leave?
It's not that Natsu and Gajeel can't leave, it's that Igneel and Metalicanna can't leave. Since Mashima made it clear that all of the dragon slayers actually were hosting their respective dragon parent, it now makes sense that Natsu and Gajeel couldn't leave Freed's Jutsu-Shiki: Igneel (and possibly Metalicanna) was alive over 400 years ago.