That was long.
But I would like to see a sequel if they can come up with a good story. I agree it has something to do with a ton of (stupid) superheroes and supervillains running around and making fools of themselves, save of course for Hit Girl. I don't know what should pull them out of retirement, but it may have something to do with Big Daddy's reputation with Red Mist figuring out who BD was and trying to reveal it. As long as she can kick his proverbial ass.
I do think that Mindy and Dave should have an evolving relationship. I would strongly oppose exploring her crushing on him (at least until a possible third installment when she'd be 16-18, at least) but a big brother/little sister thing would work. With him trying to keep her from killing anyone at school and acting sociable, while she teaches him how not to get his ass kicked in every fight he gets into.
But I don't think it should deal with Mindy having "girl problems" or PTSD. I don't want to see her questioning whether killing people are wrong, having nightmares about it and feeling unpopular for being picked on. They can touch on her being socially out of step with the world due to her brainwashing, but like in the first movie, it should be acknowledged but not the point of her character. What BD did to her was wrong, but she she should not be an angsty teenager dealing with past traumas or alienation.
I'd prefer more of her being somewhat isolated in school because when the "mean girls" try to screw with her, she breaks their noses and Dave has to keep excusing her actions. But she should remain a supporting character who exudes awesomeness, with a hint of tragedy. But I don't want to see her become a "normal teenager" like Dave and dealing with boredom, bullies, the opposite sex, etc. in a serious way. That would just take away what makes her character so likable and pushes her closer to being like Peter Parker--like Dave. While a female Peter Parker would be interesting, I don't want it to be HG.
BTW one of the three deleted scenes involves her and BD singing. I heard in an interview (it is on YT) that BD creepily started singing "We will be victorious" and then commanded her to harmonize and sing in tenor, etc. She mimicked it in the interview, it was hilarious.