Many people think that Natsu's half-dragon form in the movie Fairy Tail: Dragon Cry was END or was caused by his dragon seed. But it cannot be both as END looks different in the manga and the dragon seed which transforms a dragon slayer into a dragon has been neutralized by Igneel. Also when Natsu transforms, the OST which plays is called Dragon Force.
So was it his complete Dragon Force strength?
If we are going off of the Dragon Cry Movie, it was presumed, by loads of theorists on whether or not that was a glimpse of Natsu's Etherious mode, to be Natsu's Etherious Mode that triggered, but in all fairness, this is to be determined as a Dragon Force.
In short, no. This wasn't Natsu's Complete Dragon Force because there is no such thing as a complete dragon force. One of the best ways that support this is the Dragon Force page on Fairy Tail Wiki page,
Dragon Force greatly increases the damage done by standard Dragon Slayer spells, and grants the user access to more advanced, powerful attacks.
(from the first sentence under Abilities tab)
A better example that there is no such thing as a complete dragon force was when Sting used Rouge's Shadow and triggered a Dual-Mode Dragon Force.
I think that he had the half-dragon form because he absorbed the dragon's fire, that's why he had blue fire. Whenever a dragon slayer absorbs another element, they would probably get their power, just like how Gajeel was able to get Rogue's power and how Natsu got the lightning flame by absorbing Laxus' power.
