I'm guessing that we'll see Defenders in 2018. Here is who will probably show up by then.
Introduced in the first three seasons of Daredevil - Elektra (Season 2, revived in season 3), Echo (possibly in season 3)
Introduced in the first three seasons of Jessica Jones - Hellcat (confirmed for Season 1)
Introduced in the first two seasons of Iron Fist - Misty Knight (Season 1), Coleen Wing (Season 1), Shang Chi (Season 2)
Introduced in the first season of Luke Cage - White Tiger (Hector, dies and his daughter Ava dons the costume at the end of the season)
Introduced in films - Doctor Strange, Spider-Man
So that's around nine potential characters who aren't Daredevil, Jessica Jones, Luke Cage or Iron Fist, which brings the number up to 13.
Defenders shouldn't get a new season every year, it should be some sort of event series where it's a big deal when you get another six episodes.
And after Defenders, I'd be up for Elektra getting her own spin-off show and Moon Knight, Punisher and She-Hulk all getting series of their own. Punisher wouldn't join the Defenders but he'd still get his crossovers as an antagonist. Elektra and Moon Knight are anti-heroes. Punisher is simply a villain who hunts down other villains for sport and I don't feel like the various NYC-based heroes would allow him on the team. Elektra gets her own series first in late 2018 or early 2019, Moon Knight should be next in 2019, Punisher comes after that in 2020 and then finally She-Hulk in 2021. My reasoning is that there won't be a Hulk sequel until 2020 or 2021 to introduce She-Hulk and Elektra is already an established character so she should be the first out the gate of the second wave of shows. Moon Knight and Punisher's date would be interchangeable but I feel like Moon Knight should be first since he's based around NYC where all the other heroes are while Punisher drives across the country in his Battle Van.