So I've planned and worked on a few series of different story. And read plenty of published works to form my own philosophy on writing a series of books. Curious as to what other writers make of it.
So firstly I've concluded every character needs a definitive ending to his/her story arc. You would think this would be a no-brainer yet I feel like it isn't and quite often book after book film after film season after season characters get done to death and end up with no clear direction. I think its better if you know when you're writing to know when the arc for that character ends definitively. Sure it's sad to see them go but there's also a feeling of fulfillment with a good ending for a character. Remembering them for who they were and what they did. But never the less their story's arcs come to an end and all their conflicts are resolved. Now there are exceptions if you go back and create prequels which I have a different philosophy for.
Now it should also be noted I don't think stories in that universe cease. Just new characters with new enemies and new conflicts. Like how in history things keep going but with different people. Maybe old characters appear as side characters mentors possibly turning into the antagonist if it makes sense depending on how their arc ended.
Then building off that every conflict in that arc must be tied up. The Antagonist should either definitively die or definitively win. It shouldn't just be a conflict that never gets resolved. Like in star wars after ep 6 the empire should either die or conquer not just keep coming back under a new name. The only thing the prequel did right the republic and the CIS died and never returned. There's an exception but I can't see a lot of them. I just think the protagonist needs to eventually either obliterate or capitulate to the antagonist.
In conclusion, the arc didn't actually end if the protagonist and/or the antagonist is the same. And the arc needs to have a planned ending.
Then personally I find it interesting if every book in the series operates as its own. Like being able to read the next in the series without having read the first in the series. It's a little difficult as obviously when you're dealing with something like a Song of Ice and Fire you'll be a little lost and possibly confused if you read them out of order. But personally, I like trying to do a series in that fashion when possible.
With that avoiding major cliffhangers is a personal goal. Like maybe one cliffhanger in the whole series and that would be the book right before the last book. But in general major cliffhangers often feel like a cheap way to get you to buy the next book or see the next film. Sometimes they actually work with the plot but a lot of the times they feel out of nowhere and out of place.
Then the greatest issue I've got with numerous movie, tv, and book series is that death thrown in there only for shock value. This is usually an issue in long sci-fi and fantasy epics but when a character is killed off simply to shock you and not to move the plot forward this ending that characters arc in an unsatisfying fashion it's a bit annoying. I'm not saying never kill anyone off cause after all sometimes it works. But personally, if the only reason its happening is for shock value then I'll abstain from doing so.
So that's the philosophy on writing a series I came up with curious on others thoughts regarding it.