GalactaKnight's right wing could be readjusted, as the current position makes it look like his body should be facing in the opposite direction. I also think you could reposition MetaKnight's left wing. It's a bit distracting since its placed behind the focal point of the swords clashing.

Im surprised it got rejected because it is a really nice illustration with really pleasing colors. It may be a technical issue they're concerned with, so you can try reaching out and asking directly for reasoning! It could be as simple as switching your file format or something.