Godzilla vs. Kong is the long-delayed fourth entry in Legendary's Monsterverse. After three films, the two most famous monsters of all time are back at it again, ready to pummel one another into submission. With this being their first reunion fight since 1962's King Kong vs. Godzilla, the hype is big for this film.
However, there seems to be an edge of finality to the whole affair. Godzilla vs. Kong might be the grand finale for Legendary's Monsterverse, with no future projects being announced as of yet. If so, it would lend reason to speculate if new monsters might appear in this film to appear alongside Godzilla and King Kong, either as brief cameos, brief fights before the big brawl, or as an adversary worthy enough for the two monsters to team up against.
Skullcrawlers
Because this is a sequel to Kong: Skull Island, a return to Skull Island would likely happen at some point during the film. This means the monstrous Skullcrawlers may show up again. Perhaps this time, Godzilla will end up fighting one to spice things up after Kong: Skull Island showcased how Kong can beat down these monsters.
Because Skullcrawlers are such perfect chaotic monsters, it seems fairly likely that they will show up for a fight scene or cameo, only to play little role in the rest of the film.
Rodan
One of the surviving monsters from Godzilla: King of the Monsters, Rodan is one of the many who bowed in honor of Godzilla reclaiming his title as King of the Monsters. As such, he might now be roaming the Earth, free to run into either Godzilla or Kong. Rodan is a bit of a Starscream in the context of the narrative, switching sides to obey the stronger monster. He might challenge King Kong only to be beaten down to demonstrate Kong's strength.
What works about Rodan is that, much like in the original Showa Era Godzilla films, he can just show up wherever without upstaging the main monsters in a film. Rodan is perfect for a cameo due to his simplicity.
Mothra
Mothra very noticeably died during the events of Godzilla: King of the Monsters, but the credits show that a new Mothra egg was discovered. This might offer a possibility of Mothra showing up at some point in Godzilla vs. Kong. However, due to the monster's importance to Godzilla, it seems like re-introducing her second incarnation into the film franchise might be a little too much to develop, while simultaneously setting up Kong and Godzilla's rivalry.
It is less likely this monster will show up. However, considering the credits do hint at her return, it's not outside the realm of possibility.
MUTOs
The MUTOs appeared in Legendary's first Godzilla film, only to make a cameo in Godzilla: King of the Monsters. Once again, there are MUTOs out and about following the events of Godzilla: King of the Monsters, so a cameo is not out of the question. However, it's less likely that these monsters will appear in a full-fledged fight scene. There's a possibility one might fight Kong briefly, but the MUTOs appear pretty subservient to Godzilla after the last two films, so don't expect Godzilla to fight one.
Anguirus
Anguirus has not received much love in Legendary's Monsterverse. One of the first kaiju to fight Godzilla, all the way back in his second film, Anguirus was a mainstay during the Showa Era. The monster worked alongside Godzilla as almost a side-kick against Ghidorah and MechaGodzilla. An Anguirus cameo is not out of the question, but it's not likely.
Outside of fan service, here is little reason for Anguirus to show up. This minor monster, known for being one of the weakest kaiju in the franchise, might make too little of an impression in a film already stuffed with two iconic titans. Sure, he could be one of the many monsters who serves Godzilla and even fights Kong, but there's little he could do in-film that another established monster like Rodan or the MUTOs couldn't already do.
Ghidorah
Ghidorah was the big bad in Godzilla: King of the Monsters, pushing Godzilla to his absolute limits as he challenged the King of the Monsters to his absolute limit. However, despite a mid-credit sequence teasing Ghidorah's return, it's highly unlikely that Ghidorah will reappear so soon after his last appearance, especially not to fight Godzilla and Kong together.
If the Monsterverse continues past Godzilla vs. Kong, Ghidorah may return, possibly as Mecha-King Ghidorah. However, at the present, it's unlikely Ghidorah will appear in the upcoming film.
Gigan
Gigan is easily one of the most wild and alien kaiju to appear in the Godzilla franchise. It has hooks for hands, a buzzsaw along its belly and a bird beak. While it might seem outlandish that Gigan could appear in any capacity going forward, Gigan did come very close to appearing prior in the Monsterverse. Director of Godzilla: King of the Monsters Michael Dougherty has expressed interest in Gigan and Biollante both appearing in another Godzilla film.
While Biollante would be too difficult and complicated for this one film, Gigan might very well be in the Monsterverse's future, should it continue past Godzilla vs. Kong. Regardless, a cameo in this film would be a great set-up for a future confrontation, especially if, as in prior films, Gigan works alongside King Ghidorah.
Mechagodzilla
The only sure-fire monster to be appearing in Godzilla vs. Kong other than the two titular monsters is Mechagodzilla. This information was revealed early on, thanks to a toy leak near the start of the year. It is unclear if Mechagodzilla will serve as an opening fight or a final battle that forces Godzilla and Kong to team up, just as it's unclear who builds Mechagodzilla in the first place. It is also unclear if any other mecha opponents, like Mechani-Kong or Jet Jaguar, will show up as well.