Sorry I've not read too many of the pages here, so excuse me if this has been covered, or even if the real relationships are known (or rumoured to be known) but I had a thought about a potential answer to Rey's parentage.
Han and Laura Dern's character!
Han and Leia split, perhaps because Han was having an affair(s). It would suit his character. From what I know Dern is some kind of commander. Maybe she used to help out at Luke's Jedi Academy in the past, maybe has some Force power (or completely not, but that would make Rey being Jedi material make more sense). Maybe Dern wasn't allowed to have kids so abandoned Rey. Maybe she did this behind Luke's back, or maybe he helped. Han probably never found out he had another kid.
Lots of 'ifs' of course but just an idea for a twist. People float Han and Leia being Rey's parents but with no indication. Rey and Han do have a moment where they complete each other's sentence and it's clearly a "thing".
I dunno, just had the brainwave and wanted to post it in case I'm right!