What do you think the “colossal, undiscovered threat” will be in the next movie?

Hopefully its either Battra (Mothra's evil counterpart) or Desghidorah

The recent Call of Duty: Warzone event, "Operation Monarch" featured cave paintings of the Titans, including one that features Godzilla and a tribe fighting what appears to be a new Titan.

Interestingly, this cave painting was separated from the others, was more hidden away, and was the only "new" Titan featured (the others being Godzilla, Kong, a Hell Hawk, a Warbat, a Skullcrawler, a Rock Critter, and "Doug"), which could suggest that it has some significance. Maybe this Titan has something to do with this new threat. It's probably unlikely that Legendary would reveal the main antagonist of the movie this soon, but maybe there's a chance that this Easter egg is a potential teaser for the future.

maybe its Rhedosaurus expy from The beasts from 50,000 Fathoms 

@SasquaDash: You bring up a good point! An interesting detail about that monster is that some of it’s dorsal spikes are transparent, which makes me think they could possibly be made out of crystals, and which monster has similar crystals on their back? Just something to think about. 

@SasquaDash: Or perhaps the “crystals” on their back could be the way they are due to the heavy amount of Hollow Earth radiation the creature has consumed. Maybe this creature stores the radiations in its dorsal spines?

i think it could be   Biollante, Destroyah, or an Older "Evil" Godzilla.  In a Pre Monsterverse Godzilla Novel  Biollante came from Inside the Earth.  Destroyah doesn't have to be a Creation of the Oxygen Destroyer.   personally id kinda like it to be an Older Godzilla  weve never really seen Godzilla Fight "himself" that wasnt  a Robot in disguise  or a Black hole mutated Space clone. 

@sasquaDash  that monster there in your posted pic gives me Varan vibes  maybe a love child of Varan and Space Godzilla?

It does bare some resemblance to both Varan and Space Godzilla

Gojirafan2013, XarKhaan, Xenotaris 

It almost feels like a combination of Spacegodzilla, Varan, Krystalak and Anguirus.

I've herd some suggestions that it could be a reference to Rokmutul, who was an Anguirus-like monster from an early concept for 2014. It does resemble him somewhat, although the concept design for Rokmutul had six legs (it's possible that they could've have changed it though). I find it really interesting that, out of all the established Titans in the series, the cave painting in the Call of Duty event seems to be an entirely new creature. Maybe this creature could appear in the new movie or the live action TV series. Might be a stretch, but it could be possible. 

Good questions, i say Destroyah, Biollante, Megalon, Battra, Gigan, some new original monsters, or that cave drawing from call of duty. Interesting things u guys sayin bout it.

SasquaDash
The whole warzone thing.. It really is just non-canon, i believe COD just wanted to make their own event if the new battle pass or whatever Warzone has will sell well. Considering warzone has been dying its pretty clear that it got canned as an event. Even PUBG had one.

SasquaDash
And that new event would be godzilla vs that thing

And also why would COD dictate the story of a large hollywood movie 

Nascar 

I know. The painting probably doesn't have any significance to the next movie, I just thought it was an interesting detail.

SasquaDash
Sorry for the negativity, the new villain will probably be an og one like Destoroyah.

if it is Destoroyah they'll need to upgrade Legendary Godzilla's burning form but many will accuse this movie for being a Godzilla vs Destoroyah remake so they'll probably have to do something else.

Probably would be Destoroyah correct me if I am wrong.

Nascar

Yeah, I feel like, out of all the Toho kaiju, Destoroyah and Gigan are probably the most likely candidates for the MonsterVerse, since they've introduced both the oxygen destroyer and the idea of alien threats in the series.

the many I'm talking about is of coarse are hardcore godzilla fans. How would would you work destoroyah into the story? Burning Godzilla back in Heisei was the result of Godzilla fighting Space Godzilla and was melting down by the time Destoroyah first appears. Burning Godzilla explodes taking out Destoroyah.

Unless of coarse they have Godzilla gain a new power to defeat Destoroyah I think most fans would/would not want Godzilla to enter his burning form

Xenotaris

course* + "How would you work Destoroyah in the story" is a lazy question, the same way all the villains are. Make it so that all the damage and other stuff caused a mess tainting crustaceans and making them into Destoroyah. It's that easy, even SpaceGodzilla can be an option.

Oh and the super powers thing is easy just give him a dose of nuke and boom he has a new super power called "plot armor".