“You should not have come back"
I’m not just talking about dialogue implications, I’m talking about the significance of that duel as the long awaited confrontation between Vader and his former master since ROTS, since Vader was “the learner”, as it was always meant to be.
In Return of the Jedi, Luke asks Vader to come with him, and Vader replies, “Obi-Wan once thought as you do. You don’t know the power of the dark side. I must obey my master.” To me, that plus Obi-Wan describing him as more machine than man, twisted and evil, suggests they must have met at least once where Obi-Wan tries to bring him back and fails.
That’s what I’m referring to - I hope Obi-wan attempts to bring Vader back to the light and fails, making for a compelling story and explaining those lines in ROTJ. If the series doesn’t even explore that, it will have failed the bare minimum to justify another Vader - Obi-wan duel in my opinion.