I think it may be Nick if he survives, but not Luke at all.
Luke has never lived in Gilead, and I'm not sure that June will ever leave it even if she escapes.
She's been there for so long in such a horrific and oppressive regime, that she and the others will likely need years of therapy to recover and leave it behind.
I can imagine her forty years later, safe in Canada with two grown daughters in Hannah and Holly, probably with grandchildren, perhaps living as her mother Holly had lived, no man in her life, working for empowering women and trying to ensure Gilead never happens anywhere else, and someone making a "What a lovely day" comment and "Praise be" just slipping out of her mouth before she realises it and just for a moment she will be in Gilead again.
Nick, at least, will understand everything without having to be told, he was there, he saw it all with her. Luke will probably never understand his wife or his daughter again, not completely.
And he would have to accept and love Holly. Who he thinks is Fred's daughter and would have to be told she was Nick's, which opens up a new thing for him to come to terms with.
I don't know if June would have the patience or the inclination to get Luke through that when she has been through so much more.
He has suffered greatly by being hunted down and having his family stolen, but she's lived Gilead for years. He got out and she got caught and he won't know in the way Nick knows.
So I don't think she will reunite with Luke, if she does I don't think their marriage will last. I think it will be Nick, or nobody, or somebody completely new because she will be a different person herself if she ever gets out.