One of the great things about these comic book TV shows such as the ones that Marvel and Netflix is that they all take place in the same universe. This means that individual events from Daredevil, Jessica Jones, as well as Luke Cage, Iron Fist, and The Punisher will affect these shows. In fact, the original plan was to unite at least the first four series into one show with The Defenders. This is what is known about the new seasons of Daredevil, Jessica Jones, Luke Cage, Iron Fist, and even The Punisher, specifically what will be their big villains, especially with The Defenders.
Daredevil just finished up its second season, and it certainly had a lot of villainous action by introducing The Punisher as well as the ninja outfit of The Hand. This is a spoiler, but Elektra was killed. However, if you read the comics, you know that one of the biggest events in Daredevil's storyline was the death of his beloved Elektra, by the hands of the villainous assassin Bullseye.
Oh wait, that didn't happen on the show. However, it is pretty clear that Bullseye is going to show up, according to Ecumenical News. Charlie Cox (the actor who plays Daredevil) has stated that "I can't imagine doing a run of Daredevil where Bullseye doesn't show up at some point".
I'm going to potentially spoil something about Daredevil. In the comics, Bullseye not only kills Elektra but also Karen Page. Oh wait, there is a Karen Page character played by Deborah Ann Woll, and she has just learned that Matt is Daredevil. So, is the third season going to feature the death of Karen? We might have to wait until 2018 to find out.
As far as Jessica Jones is concerned, there is already a villain set up for that, according to GameNGuide. Will Simpson (Wil Traval) was set up as a friend of Trish, but it is revealed that he is taking a certain type of pills. It mirrors a Daredevil villain known Nuke who has three different colors of pills (red, white and blue), and wears a tattoo of the red, white, and blue on his face (not depicted on the show, yet).
Unfortunately, it is also going to be a while since we visit Jessica Jones. It is probably going to be Iron Fist that comes next after Luke Cage premieres on September 30th. Those villains are already known as Mahershala Ali will play Cornell "Cottonmouth" Stokes.
Iron Fist is coming sometime in 2017, and The Defenders, which unites all the heroes, will probably be coming in the same year. Who knows who will be the big bad villain on that show, or if there might be a group of villains.