I really like the idea of having Vader say, “Your mother once thought as you do.” This change makes sense, and not coming from a PT standpoint either. It’ll help his character be more dynamic; Vader would originally bring up Obi-Wan too often (“Obi-Wan has taught you well”, “Obi-Wan this”, “Obi-Wan that”), so for the audience to see that he actually remembers someone else in his past life makes him even more compelling. Hopefully it can be done 😃
I agree in a sense but I think it may have a diminishing quality on his character at the time as seeing him for the evil machine and right hand of the Emperor he is at the time. To hear him reference Luke’s mother and his ex lover (wife?) starts to humanise and make him relatable again (to an extent) to early on where originally this doesn’t happen until he decides to save Luke from the Emperor and the events that follow after.
Though in saying that, you do hear him call Luke “son” in the same scene as the “thought as you do” line so using “mother” could work but it also just seems an odd reference when discussing matters of the force, it does make more sense still to reference Obiwan who was his mentor in the ways of the force and would be able to detect the same feelings but obviously gave up on them long ago.
To reference Luke’s mother in regards to this actually creates questions about her that can’t be answered in the OT but can be in the PT which you then involuntary start to think about anyway as the only source of extra info on her apart from Leia’s comments about her beauty and being sad.
By the way, just had a quick look through the ROTJ script and I could only find 3 times where Vader references Obi-wan 😉 Maybe you’re memory is including times in the other movies where he mentioned Obi-wan.
http://www.imsdb.com/scripts/Star-Wars-Return-of-the-Jedi.html
“Obi-Wan once thought as you do.”
“Obi-Wan has taught you well.”
“Obi-Wan was wise to hide her from me. Now his failure is complete.”
.Val