A very rough sketch on what Flowey's omega form with the 6 human souls should be.
The Undernight version of Omega Flowey is specifically called "Nightmare Flowey"
It was, in fact, partially inspired by Titan from Deltarune, although the basic idea was still original and came to mind before chapter 4 release (and I say the same thing literally about everything else that is similar to Deltarune)
The black and white image is just a part of the sketch and the final image should have more colors and look more realistic (as much as I can do at least).
Although it is just a sketch I already feel the pain of drawing and animating it...
Now for the actual lore part, the thing that I can think of immediately is that depending on your LV the souls will actually decide if they want to help you or not. And based on which ones helped you and which ones didn't the fight will actually grow more difficult instead of always being the same.
It also means that of you were evil and no soul wants to help you then they won't fight against Flowey. Therefore to actually win something else must happen, which is currently the souls being forcefully deestroyed.