In my humble opinion, the fight does or does have to be 1 vs 1, only Natsu against Ignia, a fight that has reasons to be carried out personally by Natsu:
- Natsu was raised by Igneel as his own son, instead Ignia is the true son of Igneel, it is a fight to be the heir of the Fire Dragon King.
- Ignia wants to fight against Natsu, due to the fact that the God-Dragon of Fire knows that our main character defeated Acnologia (supported by the other dragonslayers and other characters), since Ignia's goal was to defeat Acnologia himself.
- There is a plan that Ignia has with Natsu, which we do not know for sure what it is and how Natsu would be involved in it. For now, Ignia wants Natsu to increase his powers and according to his own words to "burn everything" ".
I hope that no one interferes for this fight, even though Ignia is a target of the 100 Year Quest, the only one who has to face him is Natsu for personal reasons and also that both of them fight with full power, without nerfing Ignia and Natsu with his powers developed to the maximum taking advantage of his experience in combat.
Let's see what Mashima comes up with in this matter, I hope he doesn't do it in a hurry (although seeing how he does some things, well...), because Ignia is the opponent I'm most looking forward to seeing and it's also a very good opportunity for Natsu.