While it's known there won't be a Season 2, don't rule out Marvel's Defenders just yet as a new team and possible TV series is teased by Marvel Head of TV Jeph Loeb.
In an interview with EW.com, it's brought up that Daredevil Season 3 has a lot of freedom and didn't have to build up to The Defenders miniseries, with Jeph Loeb offering things in Daredevil could play out in other ways that have yet to be revealed. It's then remarked Defenders won't have a Season 2, with Loeb teasing other possibilities:
No, all we’ve ever said is that there are no plans right now [to do more]. It doesn’t mean that it’s off the table. And it doesn’t mean that it even has to be with the original characters that were in the first Defenders.
Loeb was then asked to further explain what he means:
There are lots of Avengers, and through that, you should be able to ascertain that there may be lots of Defenders. No one at any time referred to them as “The Defenders” — they didn’t wear a little “D” on their belts, and they didn’t go to a building that had a big “Defenders” [sign] in the air. They were four mismatched heroes who were joined by a lot of the other heroes that are in the stories that we do on Netflix, but that was that particular story.
It's rather interesting that Jeph Loeb says that basically Daredevil, Luke Cage, Jessica Jones and Iron Fist were not "Defenders," which is a pretty big statement (especially considering it wasn't all that good).
Regarding different characters being Defenders, the comics have seen dozens and dozens and dozens of Marvel characters take up the mantle ranging from the founding members, Doctor Strange, Hulk and Namor, to characters including Silver Surfer, Spider-Man, Nova, Wolverine and Iceman.
Perhaps with Disney launching their upcoming Disney Play streaming service and with the Fox Studios characters coming to Marvel, a new Defenders series will be released?