Imaculata
Hero
They probably intended for Kylo to be the BBEG, but to many people shipped ReyxKylo and Disney was desperate to please people after Solo and Ep8 failed. So they had to bring in a new one. And when you want to play it safe, whats better than to reshoot a movie people like, characters and everything?
No, they clearly meant for Snoke to be the big bad. He wasn't supposed to die a movie early. Honestly, the moral gray that both Rey and Kylo are in is the thing that interested me the most about the characters. I shipped them as well, because I want to see Rey go to the dark side, and I wanted to see Kylo Ren redeemed. I think that was the plan all along. But Disney neglected to set this all out and put it in stone. They went with the JJ Abrams method of writing, which is to leave a ton of dangling plot threads and figure it all out later. They should not have done this obviously. And they should not have given Rian Johnson so much freedom to do whatever. The 2nd movie is the most important movie in the trilogy after all.
Now the Rise of Skywalker had to redcon a ton of stuff from the Last Jedi in order to close the trilogy properly. Lots of stuff, such as The Knights of Ren, Rey's parents, Rey's visions, Snoke's backstory, are thankfully brought back. I strongly suspect that Snoke was never intended to be one of many clones by the emperor, but it was a quick fix to solve this unresolved plot thread that The Last Jedi blew up, like so many others.