WMG: Monster Hunter

Monster Hunter takes place in a low gravity world
Hunters seem to be capable of falling from great hights without getting hurt, animals can grow to immense sizes, especially invertebrates whose exoeskeleton can hold their weight with ease, compared to their Earth counterparts. The low gravity could also make it possible for large creatures to move under sand, and for hunters to carry absurdly large weapons—they simply feel lighter.
  • If that's the case, it could be a Lost Colony on a Death World. And somehow the descendants of the colonists haven't lost their Heavy Worlder muscles yet. Which would also explain how a Bronze Age civilization even has guns.
  • If you go to Area 9 of the Volcano in Tri, you can see a planet/moon that looks far too large to be the moon we all know. It's also a different colour (kind of purple), although the volcano might have something to do with that. So perhaps the MH series is set on a different world. It may just be a severe case of moon illusion (somehow), but the colour and size don't really seem to fit those conditions exactly.

Odd volcanic behavior
The volcanic areas present on the game seem to be strangely static, showing little change despite their active magma flows. Said magma flows work more like rivers (and waterfalls). The minerals present in these areas seem to be able to stay solid even at very high temperatures, which could explain why some areas remain unchanged, and the native creatures in these areas, by absorbing said minerals these creatures are able to resist the ridiculously high temperatures and some even dive in magma without problems.

Hunters are not human
The Hunters may look human but they are in fact some form of super human. They can carry and effectively use weapons as large if not larger then themselves, they can survive forces that would reduce a human to a smear on the ground, and they are capable of holding their breath for extreme lengths of time and draw oxygen from oxygenated water (passing through small patches of bubbles refill your breath meter)
  • It's implied that they're eating bits of the monsters that they hunt, so the hormones and whatnot from various giant critters are probably having some kind of effect...
    • Alternately, if we're going with the Lost Colony theory, Hunters were deliberately designed to ward off the megafauna that runs wild, kind of how some of the firelizards in Dragonriders of Pern were genetically augmented into dragons in order to ward off the Thread. Maybe early settlers used DNA from more primate-like monsters like the Congalala, Blangonga or Rajang in order to create the first Hunters, giving them the kind of endurance and strength you'd find in the megafauna. With regular interbreeding with ordinary humans, Hunters became more humanlike in appearance while still maintaining their nonhuman capabilities. Either that or they're the offspring of super soldiers who settled on the planet in order to get away from war. Alternately...
      • Alternately, they could be the result of interbreeding with Wyverians, as they are also quite human looking (long ears, toe talons, and odd aging habits aside), and are shown to be competent hunters (one piece of art shows a scrawny elder Wyverian standing over a monoblos holding up its horn), add this to the fact that most of the villages as well as the guild seem to be run by Wyverians...
The monsters are the end result of Gaia's Vengeance and the Hunters were created by humanity.
Think about it: when a monster takes you down, they don't eat you, they leave your body, thinking you're dead. That can only mean that the monsters are intelligent enough to know that humans are the most dangerous creature on the planet. Ignoring the gameplay mechanics and looking at the world itself, the monsters may have been the result of evolution, like humans kept killing and killing and only bigger and nastier animals were left behind. The more humans kept invading their territories, the more humans kept dying. The end result? Monsters that (allegedly) eat humans, thus explaining why they're so big: humans are fucking EVERYWHERE, so with enough humans, monsters would have tons of nutrition to grow big. Hunters were created to combat the threat.
  • That would at least explain why most monsters have evolutionary features that would be absolute overkill in the context of anything other that combat; a lot of monsters are designed specifically for combat (we all know it's due to Rule of Cool) but being able to shoot a water laser doesn't really do much apart from tons of damage.
The Ceadeus is based off of the Sumerian god Enki
The Ceadeus has traits of both a goat (The prominent horns and "beard") and a fish (living in the water and having a whale-like tail), both sacred animals of Enki. (Which were later combined to form the "Capricorn", which again the Ceadeus has an uncanny resemblance to) As well, both Enki and the spoiler: Ceadeus live in underground oceans. (Though freshwater in the case of Enki and saltwater in the case of the spoiler: Ceadeus) Also, the Akkadian version of Enki's name is "Ea", which is right there in the name: cEAdeus.
The world of Monster Hunter was, millions of years ago, invaded by the Kaiju
Just like in Pacific Rim, the Kaiju invaded the Monster Hunter world through a rift in the ocean floor. However, as explained in the movie, a pristine, earth-like world is not ideal for Kaiju colonization, so the rift was closed and the remaining Kaiju were left to fend for themselves. While some of the Kaiju's traits, more practical for indiscriminate killing than survival over a long period of time, were evolved out, such as their massive size, Multi-Armed and Dangerous traits and Bloody Murder abilities, some of their impractical traits, such as highly specific and energy-consuming weaponry and needless aggression remained. The result, of course, are the titular monsters of the game, which highly resemble the Kaiju. The Pseudo-Wyverns, who have wings that double as arms and run on all fours like cats, and the Cave Wyverns (The Gigginox and Khezu), who have similar attributes to Pseudo-Wyverns, use poisonous sacs and have prehensile tails, are descended from Otachi clones, while Brute Wyverns, who are dinosaur-like in behaviour and size and tend to have distinctive crests on their head that can be used in a charge attack, are descended from the clones of the Trespasser/Knifehead/Scunner-type Kaiju. Leviathans, who have crocodilian body structure and are the most adapted to underwater combat, come from Raiju clones. Elder Dragons are descended from the enormous, mysterious, Class-5 Kaiju.

Pink Rathian and Azure Rathalos are hermaphrodites.
The Rath species of monster is split into two parts: the green-colored female Rathian and the red-colored male Rathalos. This brings us to the two subspecies. Pink Rathians' dull pink scales make them look more like a Rathalos. They also change their fighting style to be a little more like a Rathalos, spending more time in the air and added fire to more of their attacks. Similarly, they lose the Rathian's motherly instincts (normally Rathians will come running if you pick up a wyvern egg, but Pink Rathians don't care about eggs at all). Azure Rathalos, similarly, is colored a little more like a Rathian and has more poison in his attacks, again like a Rathian. This is because these two subspecies are "in between" Rathian and Rathalos; they were born as hermaphrodites, having the traits of both the male and female of their species.

The Monster Hunter Frontier world is an Alternate Universe from the main series.
Of course, we do know that that Frontier development team is completely different from the main MH staff, but In-Universe this would certainly explain why that series has wildly different creatures from the home and handheld versions, despite sharing the same locales as the 2nd generation games. Differences in monster distribution and migratory patterns would have affected creature speciation, leading to the unique specimens found only in that game.

Monster Hunter is a prequel to Pokemon
Long before humanity domesticated the creatures that we know as Pokemon those creatures wandered the Earth, a constant threat to the developing humans. To defend themselves against these threats hunters began using parts of the monsters themselves as weapons and armour, eventually this philosophy gave way to directly using the monsters to fight other monsters.
  • Stories can serve as the intermediate stage to this. Society has evolved, and people learned to raise monsters as mounts. (Note that pets already exist in the main series in the form of Poogies/pigs, and pet monsters in Frontier.)

The Cheeko Sands Elder's husband is still alive
If you talk to one of the felynes at Cheeko Sands, one of them mentions finding a message in a bottle stating "Hunter with the Sword and Shield looking for Cheeko Sands." The Elder's husband is known to wield a Sword and Shield, as evidenced by the Heroic Blade. Logically, one can assume that the message comes from the Elder's husband.

Furthermore, I have a theory as to WHY he can't find Cheeko Sands. Cheeko Sands is stated to only be found by people who lose their way. The Elder mentions that her husband had an excellent sense of direction, and can't understand what is taking him so long to get there. Therin lies the problem. Since he has an excellent sense of direction, he cannot lose his way. If he cannot lose his way, he cannot find Cheeko Sands.

The world of Monster Hunter is also the Breath of Fire world, ages after Myria was defeated
There are two versions of this theory, one based off of the main trilogy and the other based off of 4's Alternate Universe, both of which involve humanity abusing technology to create biological weapons, resulting in the monsters. It's not a perfect theory, granted, primarily due to the predominantly Low Fantasy Monster Hunter universe, but some parts fit too nicely to be ignored:
  • There are many factors in BoF3 that could easily lend themselves to the Monster Hunter world:
    • After Ryu finally defeats Myria and sends her space station crashing down, whatever remained of the technology from Space Station Myria and Caer Xhan may have made its way to the rest of the world, and humanity may have found a way to create vehicles that can traverse places like the Great Desert, which eventually turned into what we know as the Sandsea (with the Oasis town turning into Loc Lac City).
    • However, this doesn't mean that monsters and malevolent dragons no longer exist; some Dark Dragons may have resurfaced to threaten the world, such as Fatalis which is shown to have a cunning intelligence—they may have only lost the power of communication over the ages due to some deleterious influence, or they choose not to speak. That, or judging by the existence of Gogmazios, some of them may have been engineered by military scientists as biological weapons, using methods similar to what Dr. Palet attempted. In this case, Myria would have been correct about humans abusing her technology for wicked ends, similar to what she described happened in the city of Caer Xhan.
    • The Brood, on the other hand, had recovered and managed to live on as the Wyvernian race, having been De Powered by the sacrifice of their draconic abilities. This would explain why they possess reptilian features. Note that their Japanese name is the Dragon Tribe (竜人族)trans .
    • Structures like the Tower, which is described in the game as battlements created by an ancient civilization to help ward off powerful elder dragons, may have been fortresses built by Myria to combat and contain the Brood. Note the "contain" part: in Frontier, there is an entire section of the Tower filled with traps and holding cells, notably the one where you fight rare creatures like Mi Ru.
    • Certain Ancestral Weapons that you get from mining Rustshards and Ancient Shards seem to possess extremely advanced technology, similar to the Chrysm-powered machines in BoF3: examples are the Obelisk Gunlance, which has visible glowing circuitry along its damaged sections, or the Skyscraper lance which has independently moving drill parts. Note that these advanced-looking weapons also possess the Dragon element, which would be lethal to Ryu's kind.
    • The Dragongems that hunters occasionally recover from elder dragons may in fact be Chrysm, which would explain why they're needed to craft certain endgame weapons and armor—they act as a power source.
  • Conversely, BoF4 appears to match the world of Monster Hunter better in some regards, since at the end of 4 the Ryu/Fou-Lu hybrid proceeded to De-power all the dragons, thus removing most traces of whatever would be considered High Fantasy from their world (although this wouldn't fully explain the seeming total absence of anything related to Practical Magic). However, note that the renegade Mad Scientist Yuna managed to escape Cray's wrath near the end of the game, which would mean that he's the mastermind behind the existence of malevolent elder dragons: he'd been reverse engineering their power as weapons of war—possibly to try and profit off of them—but they may have proven too much to control in the end, leading to them running wild and taking over many habitats around the world. That, and an actual Sandsea exists in the Eastern Kingdom.
Khezu actually does have a BGM track.
It's called 4'33".

Monster Hunter Stories will be like an awesome new Monster Rancher game that we will never have
The applicable trope is Spiritual Licensee. However, no more inserting CD's to get monsters, it'll be entirely in-game egg hunting and breeding to get more monsters in your stable.