Here are my thoughts…
I think the main plot will revolve around another monster challenging Kong’s status as King of the Hollow Earth, while the secondary plot will revolve around Kong developing his relationship with Jia, and possibly forming a connection with another Kong (assuming more Kongs will be introduced). If Kong does meet another Kong, they’ll either be another member of his species, or belong to a subspecies of Ape Titans, much like how Doug was a member of a subspecies of Gojiras.
As for the human characters, I have a strong feeling that they aren’t going to be as much in the spotlight going forward. Based on how many casual moviegoers were happy that the human characters didn’t get as much time to shine as the monsters did in GVK, I wouldn’t be surprised if the monsters take the spotlight going forward. That’s not to say that we won’t get anymore human screentime or even character development. It’s just that their stories won’t be as important as the monsters’. This is just how the MonsterVerse has developed since G14. With each movie, as the monsters get more and more screentime, they become more well rounded and personified.
I definitely think we’ll see new biomes along with new creatures. I also think we’ll see variants of the Skullcrawlers and other Skull Island creatures. After all, we did briefly see red Leafwings flying around in the Hollow Earth in GVK, so I would’ve be surprised if we see different versions of the Skyer Buffalo, Mother Longlegs, and Sporemantis. We may even get to see the Shimidah, a scrapped monster from GVK that Team Kong were supposed to run into while traveling through the Hollow Earth, along with that giant Hellhawk that was shown in the GVK artbook.
Something else to consider is that the Iwis from Skull Island originally lived in the Hollow Earth before they were banished to the surface world with Kong’s species by the Gojiras. So I don’t think it’s out of the realm of possibility that we’ll see more Iwis in the Hollow Earth, possibly even living with a tribe of other Kongs.
As for the main antagonist, I believe they’ll either be Camazotz or an original monster. If they turn out to be an original monster, I honestly don’t know what type they’ll be at the moment.