Sorry to be a stick in the mud a week later, but there's no record that Hylia was ever anything but a goddess before she reincarnated as Zelda; she didn't become one, she always was, and a key plot point of Skyward Sword hinges on the fact that Hylia's original form was a divine being.Like, we know Hylia was a person, but the idea that she became a goddess who can then transcend time to show up and effect events later in the timeline doesn't really work great for me. She can be a spirit, sure, but I like the idea of her ascending to legend status within the stories told by the Hylians themselves only, and not actually ascending to become a goddess for reasons.
In addition to that, there are several beings still alive in Skyward Sword that were just straight up around in Hylia's time; their knowledge of her is from memory, not oral tradition, and they all refer to her as "the Goddess". Demise in particular contrasts Zelda's mortal form to Hylia's immortal one, and suggests a huge gap between the two.
As another fun fact, Hylia seems to have some sort of degree of control - or at least association - with time, so timeline shenanigans aren't entirely out of the question with her.
All that said, I don't want or expect her to show up; it cheapens things in Skyward Sword a bit. I feel all the theories that TotK is some sort of timeline conclusion are all jumping the gun; they were rather baseless back in E3 2021, and while I respect the connections made for the whole timeline loop theory, it falls apart when certain facts that are seemingly being willfully ignored are considered. I don't expect Hylia to have any greater of a role than she did in Breath of the Wild, and I don't expect Demise to have one at all.