I'll just give my own personal advice from my experience, so if I repeat (Or contradict) anything above me then sorry.
First off, a story always starts out as a skeleton, then you slowly fill in all the muscle as you write. I would suggest planning out an overall goal for the arc, and the major points, but allow yourself flexibility with your work. If you try to be so rigid, then you'll just lose hair over something that you can fix with a few simple changes. Even so, not having any plan is always a bad move unless you really plan on doing heavy editing later.
Character development. Stories revolve around characters. Unless you have some perfect protagonist, always try and imagine 'how will the events of this arc change the characters?' A story without character development is boring, static, and repetitive. Furthermore, good characters are more important to the story than a good plot. If your characters are boring, then your story will be boring. Period.
Same thing with the plot. Make sure that things are constantly progressing. Plan out major developments, and make things permanent. Don't just hit the reset button every chapter like I see some stories doing, where there is no real progress.
What about the setting? Well, there are a lot of things to consider. The world, the politics, the conflicts, the hierarchy, the technology, the time period, etc. Make sure to think about all these things.
Characters? Make em not one dimensional. Don't make villains who are evil to be evil. Don't make heroes who are just saving people because it's right. Give your protagonists something to agonize over, and give your villains a reason to be villains. For random thugs and side characters who get thrown aside for a show of power, it's fine to not give em a complex personality, but aside from that it's important that they aren't just robots following something as simple as 'this person is good and this one is bad.' Give them goals, personality traits and quirks, establish their relationships with others, etc.
So uh what else? Well, I guess that's about all I can think of. Good luck.