They had the opportunity:
A) to expand on Snoke and write some proper lore
B) to expand on the “Force Dyad” and write some proper lore
C) to show Kylo as a truly broken figure who gets worse with the higher rank
Instead it’s basically wasted and feels like a sequel to movie that doesn’t exist. I mean Palpatine could have been involved too in some guise. There was a chance to finally explain the dumb super weapons and the state galactic politics. But this all requires a competent screenwriter.