As we have seen up to this point, Natsu has retained the residual energy from two of the Dragon's that granted him power. As in Igneel and Atlas.

I believe this will be the case with End's flames. That and Natsu had been referred to as a demon in 100Q. Even by the wood dragon god.

This flame will be needed to defeat Ignia as the man's own flames are clearly not good for Natsu, and the man will need something of the magic level as Ignia.

It's what happened with a lot of villians in the series, from Zoncrow to Dimeria to August to Zeref to Acnologia. Each one needed a specific form of power that either stood above or equal to their own in order to be hurt or defeated. Ignia will likely be the same as his flames don't seem like something Natsu will want to eat again.

So, I think Natsu will use a dragon-demon flame to fight Ignia.