I'm not convinced Kang was 100% FOX. While the Fantastic Four were being banned from animation and video games, Kang was still free to appear in those. So it's possible he was a shared property, much like the Watchers, who were shared and have already appeared in the MCU. So I'm not ruling out Nate and Jonas just yet.
Who's getting pushed is not entirely relevant to the MCU. Hope van Dyne was a forgotten character from an alternative universe and they ended up making her the main Wasp of the MCU. Billy's and Tommy's origins are intrinsically connected to each other. I see no reason why they would erase Tommy if they use Billy. Viv can always join later on, precisely because her comic book origin story only makes sense in a universe where Billy and Tommy existed first (she was created because Vision was trying to get over his old family by creating a new one). She can have a similar origin in the MCU after WandaVision, so she wouldn't need to take Tommy's spot.I never said they're interchangeable - but face it, who's being pushed in the comics? Not Tommy. They've never pushed him.
Kamala DOES rely on her size-changing abilities quite frequently, though, and that would be less special on a team that already has a resident size-changer. And even if she doesn't use her shapeshifting a lot, I still consider it a relevant powerset of the character, specially since it was the first ability she showcased after her terrigenesis. Plus, there's still the issue of the team having two Captain Marvel legacies, because with the Skrull/Kree conflict already established in the Captain Marvel movie, there is no way Teddy wouldn't have his origins tied to that franchise. With all of this combined, I just wonder what exactly would be the point in bringing Kamala to this specific team just so she could be a poor man's Mister Fantastic. Why would Kamala fans even want that, specially since she has her own show, which is probably going to explore and develop her story and powers much more than Young Avengers ever would? At least with the Champions she would get a team to lead and to make her stand out from the other characters.Kamala definitely wouldn't be taking Cassie's place. Cassie's one of the members we already KNOW would be used. It's Wasp who gets pushed out there, but she was probably unusable anyway for the reason I stated earlier - the existence of the adult Hope van Dyne. Kamala would probably stick to stretching anyway, more like Mr Fantastic. She rarely uses the full shapeshifting in the comics, so she wouldn't conflict with Hulkling that badly.