Thank you.
There are no bad characters, only bad writers. Carol's actually got a pretty solid foundation under her feet now. Marvel just has to capitalize on it better than they have been. Personally, I blame Marvel's obsession with movie synergy. They've seemed unsure of what to do with Carol the last few years, knowing she had a movie on the way but not knowing what it would do with her. They couldn't sync things up, and I think the publishing arm has been a little afraid to really push things because of that.
I wouldn't have a problem with "ghost Marv" floating around, giving out advice like dead-Yoda. But I dunno if Marvel could resist the urge to screw with that status quo, and I wouldn't want to give his fans any false hope of his return (which I don't think should ever happen). That's basically my only problem with the idea. On its own merits I think it's a great, fun concept; I just don't quite trust Marvel enough to handle it right.
I think there's probably stories you can tell with him (alive I mean, not as a dead man), but set those in the past. There's no need to clutter and muck things up any worse than they already are, and while Marv does have his fans, he doesn't have nearly enough to make his return a viable business choice.
I think I'd make her a secondary supporting character. Third from the top, so to speak.
Carol would get the majority of panel time and would be involved, if not the focus, in the majority of subplots (especially if this was her main headlining book and not a secondary title to compliment a solo). Genis and Phyla (is it two "l's" or just one?) would be the primary supporting characters, with the most panel time and plot-involvement after Carol. Kamala and Noh-Var would have the rest. In Kamala's case, she's got a solo title of her own and is a major player in Champions; and we're starting to see alt versions of her pop up as well, like Old Lady Khan over in Exiles (which I'm trade waiting so I haven't read it) and that new title based on the upcoming Rising cartoon. Kamala's got enough brand recognition to be a big asset to the Marvel "family" title, and she deserves to be involved, but there's enough of Kamala going around already and she's sort of an outlier in this group, so I'd rather see the focus given to other characters who need the attention more (like Marv's kids).
In Noh-Var's case, he's the black sheep of this "family" so I see him just sort of going along for the ride but not being terribly invested in it, yknow?
"We all know the truth: more connects us than separates us. But in times of crisis the wise build bridges, while the foolish build barriers. We must find a way to look after one another, as if we were one single tribe."
~ Black Panther.
I'd definitely bring back her black costume. Red sash, and mask, a must. The only thing I'd change would be to give her longer pants, or some sort of skirt like Gal Gadot in Wonder Woman - all black of course.
I think another thing long overdue for her would be a love interest. She seems like a good fit to be the Queen of the Marvel Universe, so I'd pair her up with a 'King', or at least a member of Marvel's round table. Captain America comes to mind right away. They both have military backgrounds, and I'd love to see them have their romantic getaways interrupted by an international crisis.
I'd also write a lot of stories for her in space, away from Earth heroes. Maybe make her a liaison between Earth bound heroes, with Marvel's cosmic heroes. So much history with Carol. Hoping for the best for her!