I'll take the 1000:1 on Darkhawk. I'm sure Mantis will appear, but it won't be any type of big deal type reveal.
Marvel has so many characters they can work with that it could be anybody.
Has Whedon worked on either character?
I don't believe so. Because of the large stable of characters Marvel has to work with and the way it made Guardians of the Galaxy into a massive success, I would not be surprised by anything or anybody Marvel or Whedon reveal.
Would I lose my mind in excitement over certain characters? Sure. But, would I be surprised the character was introduced? No.
I've heard the same rumour a few times and knowing Whedon is involved makes me think it's true. I suspect it's a Whedon written character(s)
It'd be awesome if he can figure out a way to work in the Runaways.
I repeat: VICTOR MANCHA!
Victor is activated at the end of Age of Ultron, but isn't seen again until the SECOND Runaways movie, the first being a surprise announcement after Age of Ultron opens.
A man can dream.