The only believable way they could make Rey Luke's daughter is if Luke got a woman pregnant without knowing OR believed she died. It's not out of the question, but I have to ask what the purpose would be for the story.
We certainly don't need to add more tragedy to Luke's story. Other than the fact it's pretty much over now, we already know the reason why he walked away, which was failing Ben (a boy who was probably pretty much a son to him anyway). Luke's already lost his kid, we don't need to make him lose another. If anything, go deeper into that tragedy with Luke and Ben's relationship in IX and how it affected them. Show us that there is still love between them, even if it has been warped and twisted by what happened pre TFA.
And if it's because Luke just didn't know that he got a woman pregnant then... okay? What does that really add to either Luke or Rey's characters other than "I guess you're my dad/daughter now". When we found out Luke was Vader's son in ESB, it added a massive layer on conflict and derailed the plot (in a good way) to something we'd never even considered. That Vader was a man and could be redeemed. Luke had already had his redemption in TLJ.
So yeah, keep Rey as Rey. I'd love to see who her family was/is and maybe even explore some mystery to it, if relevant to the plot. I think it's a stronger story with her just being a no one who became the spark of the resistance, and being part of this story because she believes it's right, and not some familial obligation.