Darth Plaguis would have been a better and new story

I like the Plagueis idea, but I do like the theme of Palpatine always being in the shadows, pulling the strings and being 10 steps ahead.

I don't think it would have take massive changes to make Palpatine fit better. Personally I'd have just put a few scenes in Episodes 7 & 8 of Palpatine giving Snoke, Kylo & Hux orders by hologram and sentinel droid (like in the Aftermath trilogy/Battlefront 2). That way, it sets up the First Order are just following Palpatine's recorded contingency plan. Then once Kylo kills Snoke and thinks he's the big boss man, you get the big reveal that Palps is still around. I'd have probably put the Rey Palpatine reveal at the end of 8 as well...because once she's out of the throne room, she literally just flys about and then lifts some rocks. Would have given 9 more room to breathe.