Actually JJ just has a healing factor. Luke is still the Tank. JJ however can resist psychic and magical attacks so if The Hood is the big bad, expect to see JJ be completely immune to many of his abilities.
Iron Fist is a healer who can help out of Luke Cage gets hurt since he has super durability but no healing factor. Granted, it takes a LOT to hurt a man who's almost invincible but he's weak against fire.
And you're also forgetting certain likely characters.
Misty Knight is the gunslinger who provides ranged support. Elektra is not only a deadly martial artist, she also has psychic powers that give her telepathy, telekinesis and precognition. Colleen Wing has chi powers but they aren't as refined as Daniel's and Hellcat has minor telekinesis but it isn't as powerful as Elektra's. And since I also expect White Tiger to show up on Luke Cage and then die and have his daughter take over in Defenders, Ava Ayala has an amulet of power that grants her peak human strength and speed, a healing factor and enhanced reflexes. Finally, Punisher probably won't be part of the team but his own neutral faction where he's just a guy with a whole lot of weapons and real combat experience.
And since I don't expect Defenders to be yearly, we'll probably get new characters introduced in the years in between. I'm predicting that we'll show Echo show up in DD season 3 and she has photographic reflexes which grant her the ability to copy anyone else's fighting style. Moon Knight has all sorts of gadgets and is also known to fight dirty. Blade has superhuman senses and agility as well as improved stamina and some level of sorcery powers. Shang Chi has no powers, he's just really, really good at fighting and I expect to see him in Iron Fist Season 2.
And if Marvel and Netflix are serious about Ghost Rider and are somehow able to pull him off on a TV budget, he has hellfire powers, sorcery and the Penance Stare.
I also fully expect Spider-Man and Black Widow to show up as well since they're both tied to the street level Marvel U.