Haven't read the whole thread, but the few theories I've heard are that A) Luke does have children, either Rey or someone else. B) The Rise of Skywalker refers to Luke's rising again. C) Kylo Ren is redeemed somehow. D) Skywalker is the new title for Jedi, and Rey is the first one.
I admit I like that last one, but every other trilogy had a "The" in front of it. Revenge of "THE" Sit". Return of "THE" Jedi. This is just "Rise of Skywalker", seeming to refer to a person rather than a group of some kind. However, B seems most likely, as apparently the translation of the title in other languages comes out "Skywalker Resurrected". But that could be a language thing.
I've heard thought that Rey is Palpatine's grandchild, which makes a fantastic image in my mind of Palpatine being a red Force Ghost, hovering behind the grandson of Anakin Skywalker, and Luke being a Force Ghost hovering behind the granddaughter of Palpatine, and the final epic battle is between Kylo and Rey, and between Luke and Palpatine. I'd love that symmetry , and Rey finding out she is not decended from some famous jedi, but from an evil beyond her comprehension, would suitably defy the trope.