Currently, it looks like Sean Young won't reprise her role as Rachael.
At this point, that's how it looks. Obviously, Daryl Hannah wouldn't be able to come back because Pris was killed by Deckard. A shame too, because of Ms. Hannah's beauty as well as her acting and gymnastic skills. But Sean Young was not killed off in the movie and was so great in it, both her fantastic acting and her absolutely stunning beauty, it seems a great pity that she won't be coming back. I don't know if she simply has no interest (that would be a real shame) or if some other reason has come along.
If the character does come back though, her role should NOT be done by another actress because that would completely destroy the audience's ability to suspend disbelief.
In case you missed the Q&A about this subject, here's what the IMDB says:
"It is at this point unknown if the character of Rachel will be in the movie. It has been confirmed by Sean Young that she will not reprise her role, nor was she asked to. With regards to the question whether Rachel could be in the movie, there are two possibilities.
The original theatrical version of Blade Runner (1982) ended with the notion that Rachel was a unique Replicant without a termination date, meaning that she could theoretically live as long as ordinary humans, and therefore still be alive during the events of Blade Runner 2049. However, director Ridley Scott famously disliked this studio-enforced ending, and in his 'Final Cut' of the movie, this addition was scrapped. This would suggest that Rachel has the default four-year life span like all other Replicants, and is long dead when the events of Blade Runner 2049 take place.
Director Denis Villeneuve has been very ambiguous about which version of Blade Runner (1982) is used as canon for his sequel, so whether Rachel will be in it is unknown at the moment. There is always a possibility that her character will be played by a different actress or is referenced somehow, but the makers have not disclosed any information with regards to this."