I can't think of another gimmick that would actually be any fun or wouldn't feel unnecessary. I loved flying, but swimming lacks any real depth (ha) and seemed more like it was added in to get more people interested in SB than actually suiting the tone of the pack. The Ruby Sea felt like wasted potential.
I'd rather we getVieramore races, character customisation options and more detailed, immersive zones than another novelty feature.