If Luthien gets reborn
Tolkien made it pretty clear in his works that everything is somewhere second hand, and not necessarily unbiased accounts. Adding that the only people who actually know what happened between Luthien and Námo are Luthien and Námo, and none had a really big social circle, there is nothing to confirm that things actually happened as we were told.
So what it the deal was actually that Luthien could go back until Beren's death, but she could not give up her immortal fate. What if Beren made her promise to give life another chance after him, to get reborn, live again and maybe even fall in love again?
So sometime in the third or fourth age Luthien feels ready and is reborn in a world were she is the mythical figure, where absolutely everyone believes she will not come back, where she is the subject if thousands of songs and very few people actually know her.
(I have to mention that this will fuck with Elrond and Celebrian very specifically, and they deserve better. Poor guys.)
Luthien meeting her father again, who almost got her fiancé killed in trying to keep her, who died over a necklace. Her mother. Finrod who died for her ans needn't have. Potentially the Fëanorians, Celegorm. Elrond. People who knew Arwen. People who believed her a fairytale.
And I'm not sure how equipped she is to deal with any of it.