I've been re-reading FT a lot these past 2 days, as I wanted to confirm a theory of mine; connected to the speculation that Acnologia was transformed into a dragon by Zeref. Well, since this is a theory thread on some of the parts that I was thinking on, namely the Dragon King Festival and its non-connection to the upcoming 10000 dragons invasion. @OP: I think I may answer to some of your questions in my theory.
So, I think, by most common predictions, we have come to conclusion that the 10000 dragons are mostly going to come from the Eclipse Gate since they are nowhere to be seen. Which led to the question, about why 10000 dragons are going to come from a time-travel gate...Well, the only place where 10000 dragons could be right now, is...at the graveyard.
My guess, is that the invasion of dragons won't contain Igneel or Graandeeney. But that the gate is going to revive the 10000 dragons who were killed by dragon slayers and are in search of revenge. Like Sting & Rogue's dragons and the numerous others slayed by Acnologia in the past.
Speaking of Acnologia, "The war that brought to the birth of the King of Dragon is called Festival of the Dragon King". That's the name given to Acnologia. If I link that with those 2 quotes""I look forward at the day we meet at the Dragon King's festival..." with "Can humans exceed dragons or is it an empty dream?", then I made a conclusive prediction:
The invasion of 10000 dragons isn't the Dragon King Festival. The meaning and significance of Dragon King Festival is when a new king of dragon will be born. It is when a human will transcend its condition and truly became a dragon. And well, when can that occur? Well, let's say that how about when a dragon slayer kill some 10000 dragons? If you connect the dot, I think we may say that the Dragon King festival is about a day when Natsu or Wendy or Gajeel will turn into a full dragon like Acnologia...Well, most likely Natsu is some kind of chosen child in that case since he's the main hero and his destiny is to save the world by transforming...
Igneel/Grandeeney talk a lot about believe in humans and stuffs like that. Maybe they actually trained humans for them, to save the world against, not 10000 dragons but against Acnologia. Personally, ever since Acnologia appeared, I had a small inch of doubt in my mind that Igneel/Grandeeney/Metalicana didn't flee from Natsu/Wendy/Gajeel due to some plans. But actually, they fleed out of fear. Maybe they all sensed the presence of Acnologia and that they were the last remaining breed of dragons. Thereby, they fleed away from Acnologia to a place known only to dragons, and that a human-turned-dragonking is ignorant of. They trained those dragon slayers in the hope that someday, one of them might defeat Acnologia. Maybe, they see the birth of a new dragon king as the only way to give rise to a savior, hence the await of a Dragon King Festival.
Finally, I have one last connection with Zeref and all those things but this one is more far-fetched. Since Natsu/Wendy/Gajeel seem to be chosen child and since Igneel/Grandeeney/Metalicana seem to be trainers of an extinct breed, perhaps they all come from the past. Maybe that's why Natsu/Gajeel got those extensive age (time-travel?!?)... Let's say that 400 years ago, Dragon Civil War was nearly ending and that Igneel/Grandeeney/Metalicana were last survivors from the wrath of Acnologia. Similar to how Exceed's eggs were thrown into an Anima, those 3 dragons took 3 kids with them and travelled to the future. For some years, they trained those kids into Dragon Slayers, in the hope that one of them will turn out powerful enough to defeat Acnologia. In year X777, they sensed that Acnologia tracked them down into the future and they fleed away. If Natsu/Wendy/Gajeel come from a distant past, we may understand why Zeref know Natsu. Perhaps he is Natsu's biological father. After all, he had tears of joy when he saw Natsu - similar to what a father could have for his son. Or another idea that since Cali pointed out about fire magic on tapestry, perhaps, Natsu is the son of human Acnologia...