A New Hope didn't dig too deeply into anyone's characterization. So there was nothing to indicate Vader *wouldn't* be central to the story either. That you may have guessed wrong certainly isn't George's fault.
.
You're asking me to a prove a negative, which is silly. If you need to ask me to prove a negative, then you are basically admitting that you are wrong.
Here's the ORIGINAL SCRIPT for The Empire Strikes Back written by Leah Brackett - the one that Lucas was unhappy with and subsequently had re-written by Lawrence Kasdan (Lucas is not credited as screenwriter for Empire).
https://starwarz.com/starkiller/201...ack-first-draft-by-leigh-brackett-transcript/
Nowhere in this script Vader does reveal himself as Luke's father.
Now what's this all about - where Luke is talking to ghost-Obi Wan and Obi-Wan says I've brought someone for you to meet: YOUR FATHER, ANAKIN SKYWALKER:
BEN
It was my time to move on.
LUKE
To move on where?
BEN
To a different part of the universe. It’s all one, if you know the way. Or rather, if you understand the laws that govern the way. Physical laws. One day you’ll understand, Luke you will. You’ll be able to glimpse, as I have, the beautiful symmetry of the perfection of this creation we all belong to… Luke, I’ve brought someone with me.
Another half-ghostly form emerges from the mists; a tall fine-looking man who approaches Luke slowly, looking at him.
BEN
Your father.
LUKE
My father.
Even with his new-found maturity, Luke finds this an awesome, not to say shattering, moment. They look at each other.
SKYWALKER
You’ve grown well, Luke. I’m proud of you.
(Luke, not knowing what to say, says nothing)
Proud, and afraid for you. You’re going into battle untried. Did your uncle ever speak to you about Nellith? your sister?
LUKE
Nellith? No. Who is she?
SKYWALKER
Your sister.
174 (cont.)
LUKE
My sister? I have a sister? But why didn’t Uncle Owen….
SKYWALKER
It was my request. When I saw the Empire closing in, I sent you both away for your own safety, far apart from each other.
LUKE
Where is she? What’s her name?
SKYWALKER
If I were to tell you, Darth Vader could get that information from your mind and use her as a hostage. Not yet, Luke. When it’s time…
(he looks gravely at his son)
Luke. Will you take, from me, the oath of a Jedi knight?
Slowly, proudly, Luke draws his light saber and activates it, bringing it to the salute. Skywalker does the same. Ben and Minch also raise their sabers, standing by as witnesses.
*********
From the Luke/Vader fight:
VADER
You’re very good, Luke. But I’m twenty years older and stronger than you in the use of the Force. You haven’t a chance with me…any more than your father had.
***************
But wait, there's more.
Here's a quote from Lucas reaching out to the
Lost creators, Lindelof and Cuse:
https://blog.zap2it.com/frominsidethebox/2010/05/lost-gets-a-letter-from-george-lucas.html
"Congratulations on pulling off an amazing show. Don't tell anyone ... but when 'Star Wars' first came out, I didn't know where it was going either.
The trick is to pretend you've planned the whole thing out in advance. Throw in some father issues and references to other stories -- let's call them homages -- and you've got a series."