I'm thinking of a method to avoid zooming in during boss fights without losing the widescreen aspect ratio. I believe the fight should be kept in 4:3 by placing barriers, like the key beams, right where the 4:3 ends, and then mirroring the missing area to both sides. In this way, the combat screen limit feels justified by the rays on both sides without feeling forced, while maintaining a rational appearance.
No, the boss won't leave its 4:3 aspect ratio, nor will your character. The limiting rays and extra graphics will only be visual, according to our vision. Even in World 2, you won't need rays; simply using stones to fill the black areas of the 16:9 aspect ratio already looks excellent. The combat arena is the same in the game engine; it's filled in for your eyes.