Tally Johnson: Things seem to be main-character driven. Like the Jack Reacher books for example. The sequential series do well but an overarching lead seems to be the key.
James Palmer: Generally speaking, series are easier to build audience interest. I'm trying to go with trilogies that end after three books, then starting a new unrelated trilogy. We'll see if it works.
Simon McCoy: Stephen King developed a fan base even though most of the time his characters appeared only in a book or two, but he was writing horror for the most part rather than adventure or pulp. I think it's more common for a reader to fall in love with character(s) or setting more so than the style the books are being written.
Evan Peterson: King really has a lot of cross-pollination of characters between his novels, regardless of whether they are stand-alone books or parts of a series.
That said, King came up in a time before everything was a series like it is now. Were he to just get started today, I wonder if he'd have the same success with the same books. The successful stand-alone novel is a rarity now, and even rarer is a second successful stand-alone novel from the same author
Richard Laswell: As a reader, I prefer stand-alone books, no matter the length. I often feel that a sequential series is more of a marketing ploy by the publisher in a bid to milk more money from a storyline.
As an example, had Stephen King not had control of The Stand, I could easily see a publisher chopping it into two or three separate novels.
That said, there is something which appeals to the human mind in the idea of linear narrative. To be able to experience a character grow into their full potential is very rewarding.
Robert Freese: I'm not much into series. I read Joe Lansdale's Hap and Leonard series but that's it. I've never had any interest in writing a series, but I am currently writing a sequel to one of my earlier books. But that will end the story. No interest in writing about these characters over and over again.
Selah Janel: I mean, comics not included, most of Gaiman's work is stand-alone, but he tends to tap into archetypes and pantheons that people are at least aware of or has really strong protagonists in his YA stuff. Andrew Davidson's Gargoyle blew things up when it came out and I don't know if he's done anything else since then. I think if anything, series get promoted more constantly because the character names etc are constantly in the public eye vs a single title which has a marketing shelf life to an extent. I think it really depends on genre, audience, and a good story as much as anything else.
Amanda Niehaus-Hard: I’ll read a good series, but I don’t mess with serial novels at all since I have never read one that was well done (apart from The Green Mile and Dickens).
They need to be stand-alones to get me interested or marketed as a long series, like Game of Thrones or Wheel of Time.
John Hartness: The horror genre is mostly standalone, as is literary fiction and several other genres. Fantasy and science fiction are largely driven by series, but then you have runaway successes like The Martian. So traditional publishing can still make best-selling stand-alones when they throw their weight behind it. But most successful indies in sci-fi and fantasy are working in series. Today. But wait six months, the industry will reinvent itself again.
Neen Edwards: I think that's why I like The Dresden Files. Each story is different and can stand alone, but I love the main character enough to read his different adventures. However, I'm not big on series in general that go on and on about the same plot. It gets boring after a while.
JH Glaze: I say screw conventional series!
Rob Cerio: The big exception to this in recent years was Ready Player One, but that book hit the nostalgia drum so hard...
David James: I think Dan Brown seems to be doing okay with his Langdon novels and he's still a relatively new author being popular only after The DaVinci Code took off a little over a decade ago. I suppose it's all a matter of the readers and the type of book. I had read Brown before he became popular, so I was already satisfied and wasn't "jumping on the bandwagon" at that point.
Series that have built over time isn't fully a recent trend, although it's more prevalent now. Think the Dune novels by Frank Herbert (especially before his son Brian and Kevin J. Anderson finished the story after his death, although you can consider those too), the Foundation novels by Asimov, even Robert Jordan began his series (which I think is really what began this current trend) in the early 90's.
Yet, Clive Cussler, Tom Clancy, Dean Koontz, and others, all had series of novels with the same characters, and even if they could be individual adventures, each one tended to flow into the next. I just love the adventures of Dirk Pitt, Jack Ryan, and Odd Thomas.
Kevin J. Anderson is an established author, and he still works hard to get out as many different novels as possible. I would recommend his Dan Shamble novels to you as a good example of something he attempted recently -- kind of along the lines of what you're suggesting - which has gained a following.
There are a lot of examples out there and others might be able to name some. I guess it depends on just how big of a following you desire.
John Gerdes: What about somebody like Kurt Vonnegut who did not ever write a series but had a lot of recurring characters?
Pj Lozito: We were all stunned at Pocket when Walter Mosley deviated from Easy Rawlins. He wanted to publish new characters, a literary novel, a science fiction novel, non-fiction, a play, YA. It didn't hurt his career in the least.