Episode 8 has its work cut out for it to fix some of the issues with episode 7. I think it’s possible that episode 8 can improve and give further depth to these new characters and stories, especially as it will have a different director and thus a different approach. After all, we learn a lot more about Darth Vader’s backstory in episode 5 than was previously revealed in episode 4. I think we’ll learn more about the relationship between Luke and Kylo that will put some of the events of episode 7 into perspective.
The problematic characters are Rey and Kylo. Rey is a little too perfect so far; they need to give her some flaws and vulnerabilities. Kylo seems too weak. Give him more training and stronger abilities, or something else that will make his character more interesting.
Regarding the new “Death Star”, I’m not entirely opposed to it (it makes sense for evil empires to want to build something bigger and badder), but they could done something more interesting with it. Perhaps the Resistance could have taken control of Starkiller Base at the end of episode 7, rather than destroying it. Then we could have had an interesting narrative about how wielding that kind of power acts as a temptation, especially if in the hands of someone growing powerful with the force. Since the base was destroyed, now we’re left with a gap that will need to be filled with some other threat in episodes 8 and 9.