good question

It seems the developer has no intention to make a new ending and will stick with his ending

Oh that’s kind of silly.

What I've read about the DLC is that it will be focused on a new land called Elrant and will be a new story set before the ending that will shed more light on the Order of Leonar and Lenne's past. So it's pretty much an added adventure before the last boss that gives us more info on the world. Some have mentioned it will be accessed through the main menu though and not a part of the main game.

But it won't affect the ending in anyway it's more like another adventure before the ending that gives us more info on the world.