Something like the fungal Orkoids from 40k would be an interesting Natural Enemy for the Versus world

Traits that make them a good natural enemy to humans (something it would be almost impossible for humans to completely defeat) and an interesting matchup for other NEs:

1. Their nature as fungal lifeforms that reproduce by releasing spores from their bodies into the atmosphere whenever they are fighting, taking damage, or dying. This makes it impossible to wipe them out by simply killing them all and ensures they will always come back to fight again in even greater numbers, unless you sterilize the planet's ecosystem completely somehow (something like the curse could maybe do this). Eventually if not stopped, they can take over the planet's entire ecosystem.
2. Their ability to subconsciously alter reality in a localized area when enough Orks nearby all believe something works a certain way. This can have all kinds of unpredictable and crazy results, since Orks tend to have very weird minds compared to most creatures, making many unusual leaps in logic, and believing some very strange things. It also ensures that their weapons and vehicles usually only work for Orks, since the only reason they work in the first place is because the Orks believe that they will work.
3. Their Weird Boy psykers, who can call on the power of the Warp and channel its chaotic fury onto their foes.
4. The fact that Orks get bigger, stronger, and even more intelligent the more often they fight and the more powerful their opponents are. This means an Ork army will grow in strength rapidly whenever they defeat an exceptionally strong enemy force, and makes sure that even when they are lose, the next generation of Orks that grows from the defeated Ork army's spores will be even stronger.
5. Unlike several NEs (Titans, Demons, Lawless, etc.), the Orks are not afraid to die fighting against a superior foe. In fact, that is what they live for and love the most, and it only makes future generations of Orks stronger.
6. Orks have amazing immune systems and are extremely resistant to all manner of diseases, poisons, supernatural corruption, and similar things. They also are almost completely tireless.
7. Their vehicles such as the Mega Gargant mechas and Savage Gunships are really huge and powerful.

Matchups:

1. The Neo-Humans' cruelty might actually work against them in this case, because the longer you hurt an Ork before killing it, the more spores it releases, multiplying the number of new Orks produced. Also individual Orks are probably a good deal stronger on average than individual Neo-Humans, and will rapidly multiply to the point the Neos would have trouble not getting overwhelmed in spite of their superior co-ordination and intellect.
2. The Parasites might not be able to infect Orks at all due to their insane immune systems, and even if they can, some of the larger Orks will probably be able to resist the infection. Parasites that infect very powerful creatures like the larger beasts of the Gaia's Wrath world, very strong Titans, or Demon Lords would be difficult for Orks to fight, but stronger Ork Warbosses or large groups of weaker Orks supported by vehicles could still probably win those fights.
3. All the fodder Demon Soldiers would be toast, the Demon Executives and/or very large armies of the weaker Demons would be worthy enemies for a medium-sized detachment of Orks, but I think only the Demon Lords themselves supported by various weaker Demons would be able to fight an actual Ork army, and even they would be in trouble if Mega-Gargants or Gunships were there. Idk what would happen with Byakuo, since he hasn't done anything yet in the story.
4. A group of Madalans would probably wipe out a small army of Orks, but they wouldn't know (at least not at first) to completely obliterate the Orks' bodies before they can release spores or to incinerate the area so the spores can't drift to other parts of the world. As a result, the next generation of Orks would be much stronger and smarter because of the encounter.
5. Based on what we've seen the Curse is kind of a hard counter to Orks I think. But I kind of feel that way about most NEs, so the Curse most likely has some weakness we don't know about yet.
6. The Lawless should stay far away from the Orks if they want to stay alive. Orks don't have brains to control with tech (being fungal lifeforms), the Orks' technology and abilities also can't be stolen and used by them, and the Orks are much stronger than individual Lawless seem to be and almost always show up in large hordes.
7. The Orks probably can't take down a Kaiju (unless they get stronger over several generations), but they would certainly enjoy trying and would keep coming back to annoy the Kaiju over and over.
8. The Orks have their own extremely powerful gods, Gork and Mork, and they are rarely able to be corrupted by Chaos in 40K so they are unlikely to be convinced to worship the Mad God. Idk anything else about what the Mad God does, but he probably is a bit above the Orks' paygrade for fighting, although they will certainly try.
9. Gaia's Wrath is an Ork's paradise. All kinds of new, interesting opponents to fight around every corner that will almost always attack the Orks (since the beasts mostly view human-shaped creatures as food). Eventually though, I think the Orkoid fungal ecology would start to replace the natural plants and wildlife of Gaia's Wrath and destroy the forests.
10. Lone Titans like the one killed by fodder Demons or small groups like the ones the Lawless captured would easily be killed by Orks. Mid-sized groups of Titans like the ones that attacked the Neo-Humans would be more of a challenge but still probably lose. Larger groups of Titans, especially if led by Gimbak or some of his sons, would be a worthy challenge to an Ork army.
11. The World Tree would probably be a well-matched opponent for the Orks, since just like them, it is a world-spanning network and can always grow back more. Idk how this matchup would go beyond that, though. Need to see more of how the World Tree works.
12. The AI army would probably be a good counter to the Orks as well if they were able to figure out the Orkoid life cycle, since as robots, they don't need the world to remain habitable and can actually afford to just sterilize the environment.