Here is a part of the interview with the Dark Empire author.
TOM: Our original proposal was to bring back Darth Vader’s costume and mask, with somebody else inside it. We felt the Empire would want to maintain the fearsome image of Vader and wouldn’t much care who was wearing the armor and breathing mask. George vetoed that idea (although he did allow us to have Vader appear in dreams and memories). He said, no, you can’t bring back the Vader costume…but if you can figure out how to bring back the Emperor, that’s o.k.
The obvious way to bring back the Emperor was with cloning, which George immediately approved.
Now, Zahn has said in an interview that bringing back the Emperor goes against the story of Return of the Jedi, where we see the Emperor destroyed by Darth Vader. But Zahn misses something essential about that scene: When the Emperor dares Luke to “strike me down”, he seems utterly indifferent to his own death! He feels that whatever the outcome of this confrontation with Luke, he, Palpatine, will conquer.
Yeah, a clone of a character isn’t the character itself. And it’s in a book (no longer canon). Not in a movie Lucas made.
Interesting to read that Lucas was against the idea of bringing a (lame) wanabe Vader’s figure. Something Kasdan and JJ eventually did (Kylo Ren, lame in TFA, way better in TLJ), which was a very bad idea Rian tried to fix (“remove this stupid mask”); but sadly JJ is directing again for the finale 😕