I think having her be Luke's daughter would be too boring and safe, plus the idea of only one family of Jedi that matters in the entire galaxy is pretty lame.
People are having a hard time imagining Rey being Luke's daughter could be handled so well in Episode VIII (or dare I say IX) that it was worth the wait to find out for sure. Like, I've said that if they do this, it needs to be some incredibly poignant moment that makes the wait feel like it was worth more than a dime, and they were already smart enough to not treat the Ben Solo thing like an
earth-shattering twist. Instead they smartly revealed it early and didn't treat the audience like they're gullible to every twist they could try and do. And getting that fact early let the film's characters develop in more interesting ways: instead of the twist being that Kylo is their son, it became about how Han and Leia were distraught and trying to bring him back, which led to Han's death-- which
was a powerful moment.
Vader being Luke's father was a twist, because there was absolutely no evidence until that movie, and even then it still blew people's minds because the sheer idea of that was crazy at the time. Rey being Luke's daughter wouldn't be crazy or unpredictable, just as Kylo Ren being Han's son isn't crazy or unpredictable either. The way they played the Solo thing was perfect, but holding out another movie or two just to say "yeah she's Luke's daughter" without some
mindblowing reason or poignant moment behind it would feel, as you said, incredibly boring and safe. And they may well have that mindblowing or poignant moment all lined up, but the above is why, at least for now, I don't think they're gonna pull that on us.
They've already did the "omg he/she is his/her son/daughter" with Kylo Ren. We should probably be hoping that they do something more unique with Rey. I thought the Solo thing was great and handled very well, but they've already did this. Do something more interesting with the protagonist.