What may have seemed like a senseless act of violence in The Defenders was actually a major Marvel moment. Major spoilers for The Defenders ahead. What happens to Misty Knight's arm in the comics? Bakuto sliced it off in the finale of the Netflix series, but the Marvel comic origin of this impairment is a little different.
The original injury resulting in Misty Knight's missing arm had to do with a bomb at a bank. She removed the bomb, but it still detonated, taking her arm off in the process. There were bombs present in The Defenders, but her amputation was a bit cleaner. In the comics, Misty is treated and given a cybernetic replacement arm at Stark International — that's right, Iron Man himself upgrades Misty's punch. Tony Stark recognized Misty's heroism and gave her a miniature super suit. Unfortunately, the injury prevented her from field work at the NYPD, and Misty quit to become a vigilante rather than take a desk job.
Misty's first robot arm simply gave her super strength and a killer grip. Later versions included vibranium, the metal that Cap's shield and Bucky's arm is made of, repulsers, an energy beam that can ice things, magnetism, blasts, and technopathic powers.
It doesn't seem likely that Robert Downey Jr. will show up in the second season of either Luke Cage or Iron Fist, as cool as that would be. However, Colleen does allude to Danny having used his connections through a hospital that he owns to get her the best possible care. Either he is the stand-in for Stark, that aspect of the comics will be ignored, or it will be vaguely alluded to the way characters in the Defenders universe talk about "the Incident" and "the big green guy."
At least it finally happened. Misty had an injury in Luke Cage Season 1 that hinted at her bionic arm, but didn't follow through. Now she's one step closer to becoming a full blown superhero instead of a by the book sidekick. Whatever happens next will be cool to see on Netflix, as none of the heroes thus far have truly been technology-based. Fingers crossed for that Stark cameo, just in case.