I previously shared my thoughts on the first two Marvel shows coming to Netflix, so here now are my thoughts on the back half of the four full shows Marvel's making...
Iron Fist: This is the character I'm most excited to see in live-action. Martial arts is such a visually interesting (and sometimes downright captivating) thing to watch, that every time I read a comicbook with Iron Fist in it, I can't help thinking about how much more cool he would like with the benefit of full human motion. I think we've come to a point in film history where Western movie-makers can give a pretty solid version of the lightning-fast action we see in classic Asian kung-fu movies. We can already see some hints of this in other superhero adaptations like Captain America: the Winter Soldier and Arrow, where the fast-paced fight scenes are more than just the old uppercut or right cross. I'm hoping they really take that a few steps further, and do everything they can to give Iron Fist as much of an authentic martial-arts feel as possible.
Personally, I think it might've worked better to release Iron Fist's and Luke Cage's series at the same time, to take maximum advantage of their team-up potential. As it is, if we assume that Cage's heroic origin will be in his own show, then he probably won't be appearing at all in Iron Fist. Fist can still cross over into Cage, but I'm sure they'll want to give Luke a little breathing room to establish his own individuality, and there might also be other heroes guest-starring in that show, as they ramp up to the ultimate Defenders crossover. So it does feel like the Heroes for Hire duo might have had a lot better chance to soar if the two series had been released at the same time, but I guess it's just one of those things. Of course, if all these shows do well, we certainly can't rule out the possibility that an additional Heroes for Hire show could happen in the future.
As for the casting, I vote Iron Fist the hero most likely to be played by an unknown, because his martial-arts abilities are so central to the character, I don't know that they can get away with casting some pretty boy and just having him train for a few months. They'll probably need someone with preexisting martial-arts experience.
Meanwhile, Marvel could win more points for diversity by making girl detectives Misty Knight and Colleen Wing integral to the series. I, personally, am not aware of any other major supporting characters for Iron Fist, so it seems very likely that they'll be in the show, it's just a question of how they'll be used. So far, Marvel's seemed very intentional about bolstering the contributions of female characters, so I'm pretty confident that Misty and Colleen will have plenty to do here.
Luke Cage: I think the elephant in the room on this one is that Terry Crews is clearly the most ideal choice for the role. Sure, he is 46, but both Iron Man and Iron Patriot are older than him, so I don't think that should rule him out necessarily. What might be a more prohibitive factor is his schedule, since at this point he has a regular job on Brooklyn Nine-Nine and is the new host of Who Wants to be a Millionaire?, as well as the likelihood of appearing in any further Expendables movies that might be made. If he could possibly find a way to fit it into his schedule, it seems likely he'd do whatever he could, as he's previously said it's something he would be privileged and honored to do.
If Marvel doesn't end going with Crews, I think whoever else they get would be a pretty tough sell to fill his theoretical shoes. In some ways, Crews is such a perfect casting choice for Cage that casting someone else instead of him might almost have the same jarring effect as recasting a character in some people's minds. Crews already embodies the character perfectly, even without ever having played him. Anybody else who may get cast will still have to live up to his standard. It would take a lot to equal his mix of physical presence, charisma, and acting ability.
An interesting postscript to the casting discussion is that Jeph Loeb said in the Daredevil panel at New York Comic-Con that Luke Cage had not been cast at that time (nor had Jessica Jones). So when Idris Elba said in August that he thought the role had already been cast, he must've been mistaken.
Meanwhile, in terms of the story, Marvel's already starting to lay the groundwork for his world with the "All Hail the King" one-shot taking place in Seagate Prison, Cage's pre-heroic place of residence. Rumor has it Ant-Man will revisit that locale, which could actually mean that prisoner Carl Lucas may have a cameo in that movie (which would obviously give us a pretty big clue about who will be playing him in the series), but that's all speculation for the moment.
Cage's series will probably be the most fun in terms of cross-continuity, because it can play in the more full sandbox of the characters and storylines that have previously been introduced. While it's certainly possible that any of the heroes could have some cameo or minor appearance in a show that debuts before their own, it seems like the most probable expectation is that their heroic origins will be on their own show (though this is by no means a guarantee, depending on how closely this version of their origin might be tied to another of the heroes), so the crossover for the earlier shows will probably be more limited. But with Cage being the anchor series, it will have the fullest ability to make use of all the interconnectedness and crossover potential that has made the Marvel Cinematic Universe so fun. Who knows, if Cage is indeed making a cameo in Ant-Man, it may be that Ant-Man will make a reciprocal cameo in Cage's series. Anything's possible, especially when all the other building blocks have already been put in place, so I predict that Luke Cage will be the most diverse and colorful (pun recognized but not intended) of all Marvel's Netflix series.