Given how the timeline has showcased a multitude of different locations and accompanying dates, I'd say that the sequel will likely visit a few different global locations.
As for the OP's questions, I'd say that Mothra and Rodan will be dangerous to Humans much like how Godzilla was: unintentionally causing destruction in the pursuit of some other goal, without any malicious intent.
As for Millie Brown's character, it's hard to say for certain. Given she'll likely be one of the leads, and given Mothra's general character over the years, it's not out of the realm of possibility that she'd communicate with the giant moth in some capacity, somewhat like Kong's relationship with the Humans he meets.
As for King Ghidorah, I'd say that his beef would most likely be with Godzilla for the most part, considering the two have tussled in the past. The two would be focusing on each other for the most part, with King Ghidorah likely being the more chaotic and destructive of the two rivals. I'd say that he probably wouldn't form any real interactions with specific Humans, probably just going on a killing spree of them in general.
