When I wrote my first book, Lifehack, I intended it as a stand-alone one-off, (OMG, I R RITE A BUK???) but it had started a world I could tell other stories in. Watching Yute and Echoes or Erebus follow the same world, and have some recurring characters, but have different feels to them.
Lifehack was pretty 'hollywood', with a couple cities worth of casualties and fun over-the-top weaponry here and there, and a big happy ending. Watching Yute was intentionally a downer, with smaller battles and far smaller, but much more personal deathcount. Echoes of Erebus was somewhere in-between, but was pretty heavvvvy sci fi with the main character not even being human. And I call those 3 books a series. They DO have a continuity for that world and while they're readable separately, there are aspects that impact the next book.
But as a series, it wasn't well planned out.
Then we have the Rubberman series. I'm working on the 3rd book, and still following, more or less, the plan I made for the series. They're still readable separately but are much more tightly linked, thematically, and in the characters involved.
I had a plan for at least 4 books in the series, with an open end that could lead into other things, but as I chip away at book 3 of Rubberman, I find the concept set for 3 might not provide enough content for a book...
Which means, book 3 might end up being the original concepts for 3 and 4.
Which means, "Rubberman's Blackout".... might not be the title...and that bigger things will happen in it that was planned for book 3. It COULD be the end of the ... initial series.
As for what happens in the wake of the events of book 3/4?