About the Hand's master plan being lame? The ability to live forever seems like a legit plan to me.
Yeah, they'd clearly already lived for centuries (Alexandra was constantly name-dropping people long dead, for instance, or that she'd eaten a dish like that in Constantinople, or whatever), resurrecting as necessary with 'the substance,' and now they were out.
Alexandra using the last of it on Elektra was interesting because it *seemed* like 'the substance' (aka, dragon marrow?) was no longer rejuvenating her body the way it should. If that was the case, even a fresh dose of immortality stuff might not have helped her, in which case her choice to use her last dose on Elektra might make sense if she saw Elektra as her replacement... Or maybe she had a very specific intent behind calling her a 'vessel,' and was planning on destroying every part of Elektra that was a free-thinking person, and moving her own consciousness into her body, somehow? Hard to say. Unlike Gao, who was showing off her super-power to force-push people, we didn't really get to see Alexandra in action.
But I do think it would have been more interesting to have the door underground lead to a secret entrance to Kun Lun, and they could still have the dragon remains there.
After reading the Immortal Iron Fist run, that's exactly where I thought they were going. But I suppose there's no reason why there couldn't have been dragons on the North American continent, back in the day...
The Black Sky. What exactly made them so special? It was obvious that Elektra was more than human here but she was still the Black Sky in DD S2 and she was not at the same level as she is here, so why the change after her revival?
She seemed to be more 'pure' Black Sky after her revival (and we already know from DD season 1 that Stick generally hunts down and murders Black Sky children, considering them inhuman monsters, so her being raised by Stick might have led to her being trained in such a way to discourage any of her Black Sky potential, as he tried to beat the Black Sky out of her).
As for what they are, I think they are just mutants. Marvel can't say the 'M-word,' but the name 'Black Sky' evokes images of the skies after Hiroshima/Nagasaki, which, in a superhero universe, could have led to a generation of people affected by radiation and producing children with superhuman potential.