Rowling
Actually, it works because Rowling is a blasted genius.
The rules do not always apply to everyone and every novel. But had that series not been popular, readers would have been content with the way the first novel ended.
The thing is, you can't just skimp off at the end of a first novel, intentionally or otherwise. Rowling didn't. There was certainly much more to tell about Potter's world, but there always is, IF the novel is popular. Unless your hero grows up, grows old, and dies in the first novel, there's always room for a second and a third and a fourth.
But if the ending is unresolved in a writer's first novel, the editor simply isn't going to care about explanations. Unless that novel sells very well, there will be no sequel or series, and it will be a double flop.
I think you're looking at this is reverse. For a new writer, the editor lets you know, you don't let the editor know. No matter how a novel is written, even if the writer has every intention of making it a series, odds are extremely high that that novel will not sell well enough for an editor to want it to be a series. It's just not a chance an editor is willing to take, except in the rarest of circumstances. There's far too much money at risk. And losing money means an editor may lose his job.
The Rowling model might actually be the right one. It certainly left readers wondering what would become of Harry and the others, what further adventures awaited them, but it was still a completely standalone novel, with absolutely no need of Rowling mentioning anything about a series. It became a series solely because it sold so fast they couldn't keep the shelves stocked.
At this point the editor begged Rowling to turn it into a series.
This is how it almost always works for new writers. You need to write a standalone novel, which means one that will satsify the reader in every way, even if a second novel is never written, because odd are it won't be, whatever the writer's intentions. The editor knows this, even if the writer doesn't.
There's simply no gain and much potential harm in writing a first novel with an open end, with unresoved issues relating to that story, and in telling an editor you have a series in mind. Every new writer seems to have a series in mind, and this simply is not what editors want from a first novel by a new writer.
It's fine to write a novel, as Rowling did, that ends with a protagonist young enough to have many more adventures, and who lives in a world obviously capable of providing those adventures, but it isn't fine to skimp on the ending because you want the novel to be the first in a series, or to tell the editor it's going to be part of a planned series. She not only won't care, it's exactly the opposite of what she wants.
First sell a novel. Then worry about sequels and series.
It may seem that series are common, but compared to the number of novels being published, they are quite rare. Far rarer still is the new writer who gets anywhere by mentioning a series up front. It really does make an editor roll her eyes and mutter, "Oh, God, not another one."
Sell a novel, worry about the rest later.
