What is happening on Netflix with the Marvel series has never really been done before, unless you count the DC/CW shows. The plan was having Daredevil, Jessica Jones, Luke Cage, and Iron Fist, and then blend them all together for The Defenders. So far, so good, and Daredevil even has a spin-off show in mind with The Punisher. That's five shows taking place in the same universe, and a villainous ninja organization could tie all these shows together.
As far as Daredevil Season 3 is coming out, don't expect to see it until 2018, according to Ecumenical News. The issue of Marvel's Netflix's series is that it takes a while to make these Netflix shows, and next up is Luke Cage (set for September 30), and Iron Fist (set for some time in 2017) are next on the Marvel Darker Cinematic Universe dance card.
Chances are, The Defenders series, which is supposed to unite all four heroes, is being filmed soon, if not already. It would not surprise me if the main villains of The Defenders and possibly Daredevil Season 3 is The Hand. This ninja group was introduced in Daredevil Season 2, and it seemed to clash with the style established on the show, especially when The Punisher was also introduced. There was a lot of talk on Daredevil that The Hand has this plan to attack the city with a huge pit, and perhaps Elektra and her possible resurrection will play in to that.
As far as Jessica Jones is concerned, it is possible that she will also be delayed until 2018 as well, according to MNR Daily. If you watch this program, you will find that it ties into Daredevil very little, but that will change with The Defenders. Jessica had her hands full with Kilgrave in Season 1, but without revealing any spoilers, the villain of the first season is not returning for Season 2.
Luke Cage, who was given a proper introduction on Jessica Jones, will also has his hands full when he takes on crime head-on in Harlem. There isn't really much known about this program as far as how it might tie-in to the whole The Defenders series. At this point, Daredevil and Jessica Jones have had some carry-over into each other series, but still remain very distinct. Luke Cage also looks pretty unique.
As for Iron Fist, there was a trailer at Comic-Con that didn't show much about the martial arts hero. By the way, it really looked like Daredevil Season 2 was setting up a lot of mystical martial arts stuff that will be seen on Iron Fist, not to mention The Defenders.
So, what will keep viewers watching the Marvel Netflix series in the future is if all these stories tie together, and tie together well. It really all comes down to the writing, and it is being handled very well from what is seen of Daredevil and Jessica Jones. It is possible that perhaps Netflix and Marvel could really get their writing act together, and plan to produce four Marvel shows each year, one per season. As it is, it is set up so these Marvel/Netflix shows can only appear once every two years per season.