What if Rey joined Kylo Ren at the climax of TLJ?

No, disagree.

Because the sacrifice of Luke was one of the best thing that I saw in the SW movies, in every single way. Rian Johson made him a legend. His sacrifice will inspire thousands of worlds across the galaxy.
RJ made of Luke the ultimate and most important jedi ever.

 

Mr Plinkett had a point when he said that all the Crait battle was not necessary and that it would have been a complete different movie if it has ended at the moment when Kylo asked Rey to join him.

I definitely agree with him.

 

Cliffhangers for the sake of cliffhangers is boring.

 

I don't think that works because, simply put it isn't Luke's story.
If this were actually about Luke Skywalker and not the greater Skywalker saga, sure that could work with a decent amount of other re-writes as well (because Luke's arc is essentially the crux of the ST in terms of theme)

But it's about coming to terms with past mistakes and making sure the next generation is set up to be better than you were.

"Pass on what you have learned. Strength. Mastery. But weakness, folly, failure also. Yes failure most of all. The greatest teacher, failure is. Luke, we are what they grow beyond. That is the true burden of all masters."

If Rey really falls to the dark side in anyway, then Luke truly failed. But she didn't. She's growing beyond Luke the same way Luke grew beyond Yoda and Obi-Wan.

While you aren't wrong it works for more talk and speculation, with everything else the movie presents, it doesn't work thematically; which I would argue is more important for Star Wars. And I would argue that TLJ actually has themes and context to the previous films that TFA doesn't have or at least beyond a superficial level. So changing that largely leaves the ST without any themes or attachments to the larger legacy of SW outside of basic level things.

And I'm sure a poster like @Jayson who has posted several times about the themes and mythology of Star Wars in amazing detail, could elaborate on it (if he even agrees that is)

 

I think this would have been very interesting. ESB ends with them talking about the next journey, and that they will rendezvous on Tatooine etc.

Everything just kind of ends with TLJ. There's nothing to leave us to look forward to.

 

But she did say to him 'Ben please don't go this way.' And he refused--not only that he insulted her by calling her 'nothing' implying that he was royalty and she was just riff raff so he was doing her a favour, it's insulting and belittling. , So I don't think it would have crossed her mind to press it. Also remember he was in a rage and seemed dis-inclned to listen to reason. Or to be blunt, he was essentially behaving like a child having a tantrum. If you've ever watched a child having a tantrum they don't tend to be cooperative. Also Rey's young and a bit naive in that department. If she had been older, more experienced with men (or people in general) and mercenary she could have joined him and ruled the galaxy through him. Given how infatuated he is with her.