Marvel applied its Avengers formula to television as they went with a decision to have four separate television shows introducing audiences to Cinematic versions of comic book characters, and then combine all of these shows into a movie or a limited series with The Defenders. The first character to be in this group was Daredevil, and his Season 1 such a hit that it didn't take too long to green-light a Season 2. Other shows are planned like Jessica Jones, premiering on Netflix on November 20, 2015, and Luke Cage sometime in 2016. The big question mark is Iron Fist, who might not be in the Defenders universe after all.
The reason for this supposition about Iron Fist is that there has been a lot that is known about the first three players in the Defenders universe, but very little has been revealed about Iron Fist. After all, Netflix audiences have already seen Daredevil fight crime in Hell's Kitchen, and he will be back in 2016 for another season, taking on comic book inspired antiheroes like Elektra and The Punisher.
The tie-in to Jessica Jones is that it also takes place in Hell's Kitchen, and even though there hasn't been any word on a definite crossover episode or episodes, there is one character who will be present on Season 2 Daredevil and Season 1 Jessica Jones. This would be the Night Nurse, aka Claire Temple, played by actress Rosario Dawson. This character helped patch up wounds on Daredevil, and it is possible that she could do the same for Jessica Jones.
Jessica Jones also has the main character for the next Marvel superhero series in its cast with Luke Cage. Mike Colter will play the African-American superhero who has increased strength, just like Jessica Jones. The natural link for another superhero would be Iron Fist, as he was a partner to Cage in Marvel comics in the seventies and eighties, back when Cage called himself Power Man.
A few months ago, CinemaBlend revealed that there was "trouble" with Iron Fist, and the fact that very little is still known about this fourth Defenders/Marvel/Netflix series could mean that this fourth series has been abandoned, for now.
The problems with the idea relate with the origin of Iron Fist's powers, which are tied up into mysticism. So far, Daredevil Season 1 went for gritty realism, but Jessica Jones will introduce characters with definite super-powers. The issue is the tone that these shows carry, in spite of taking place in the same world as The Avengers, might not allow for mystic powers.
If Marvel wants to cut off the Iron Fist show, now would be a good time. There is already talk about giving The Punisher his own show, and perhaps Marvel is waiting for fan reaction of the popular Marvel vigilante from Daredevil Season 2 before they commit to that decision.
If Marvel/Netflix doesn't end up using Iron Fist, they have several other "darker" characters that would fit the tone in the Netflix Marvel universe like Moon Knight and other characters that lack superpowers and take their fight for justice to the streets.