I've been playing a lot of Monster Hunter during the quarantine to help pass the time. I thought up some weird weapon ideas, and including some overpowered D&D stats to each too, because why not- not like we have anything better to do right now lol
Stats- Ranged, Deals 3d4+6 Damage
Probably the most well fitting of all of these ideas. Monster Hunter World introduced Slingers, which let everybody in the game have a small ranged and support option. Monster Hunter World already has three ranged weapons; The light bowgun, heavy bowgun, and the bow. It would be nice to get a sling in there, to add a blunt ranged option.
Firstly, I imagine this great sling as being a long strip of fabric which lets the Hunter spin really over sized boulders and the like over their head. This fits with the rest of the Monster Hunter aesthetic, as every weapon in the game is already hugely oversized to fit with the theme of fighting giant monsters (and also makes it easier to actually hit them). So the Hunter can use this as a ranged option, perhaps with slingers still being common tools everyone gets or not, this fulfills a different niche.
Firstly, it can be spun around and deal blunt damage much like a higher reach but lower damage version of the hammer weapon. As in, it deals bonus damage and helps stun monsters if you hit them in the head. The Great Flail can be used like this but its main ability is to launch heavy iron balls at the monsters. Different ammo types, like a caltrop type that drops nails everywhere for the monster to step on, explosive bomb type, poison gas balls and so forth are all potential ideas. Perhaps the sling comes with a free “unlimited ammo” ammo type that looks like generic rocks in the level. It's implying to not actually be a type of ammo you're carrying with you, moreso just random stones you loaded up to hit monsters with.
I really like this weapon idea because I can imagine it very clearly; the weapons in the games already have a great aesthetic design of being mixed with metal and/or the monster's body parts, scales, feathers, and colors. This sling really works well into that; just imagine a hugely long strip of a monster's colorful scales being used as a weapon, with a hard leather or bone pad at the center where the ammo is stored, to give the look more variety. I think this weapon would need to be balanced with the others; especially since the hammer is kind of hard to hit enemies in the head with but pays off with its high damage and stun. So this flail could hit enemies in the head but be much weaker then the hammer to compensate for it.
Stats- 2d6+2 in Pike Form, 3d10-2 in Scythe Form
This weapon appears as a long Pike, with a slightly curved blade at the end. When held would have a unique weapon pose in the game (held in an overhand grip, reminiscent of poking and prodding at giant monsters). I think we need a big long spear in the monster hunter games, since you're hunting huge monsters but yet spears seem strangely absent. This weapon would have a decent to long melee reach, letting it poke at monsters with small damage attacks, but consistent damage and at hard to reach places, similar to the lance. You could also give it a small spring-loaded section near the head of the spear, to explain its extra long reach better; imagine a thrust attack combo where the end of the weapon extends even further to perform an attack.
However, this weapon has a second form. Much like the Charge Blade or Switch Axe. It can turn from a long pike which is more defensive into a scythe; the blade snaps at a 90 degree angle from being straight ahead. It loses much of its reach, but can now perform long and sweeping cuts and slashes, mostly horizontal due to how long it is. This isn't useful for a lot of types of monsters, but is useful for some, so its a good trade off. This scythe form could be its primary damage-dealing form. Perhaps the weapon has a heat or “coil” gauge. The longer it spends in this form it charges up, the explanation being the springs holding it in this position are getting too hot. If you overcharge it, your attacks are deflected or you are forced back into the first form. The idea is to use this form when you want to deal a lot of damage, and use the other to sustain yourself in a fight more.
Second idea; much like above, but instead make it a positive instead. Slashing with the scythe form builds up meter and, when in pike form, can be used to perform a super strong drill attack with a long wind up. It would fit the Monster Hunter aesthetic with lots of other weapons.
 Circular Saw
This weapon looks like a giant circular saw, or maybe pizza cutter without the handle. It's got a short range, something like the hammer or dual blades in game, and is meant to be a high power, high risk weapon. Basically all of its attacks are powerful cutting attacks; the sawblades spins and deals damage up close. The more attacks you deal, or perhaps with revving it up using the special or power button (for whatever game system or controller you're using), it deals more damage as the blade spins faster and faster. At max power it deals several hits with each swing, the bonus hits are from the rapid attacks and I imagine this weapon is really strong against monster legs and/or breaking off monster parts.
Out of all the weapons; this one seems the least “realistic”, but to be fair monster hunter has always had this sort of pseudo-mechanical fantasy weapon thing going on that I've always liked, even before I played it. I don't think it would be out of the question to have a power saw style weapon, even if its made of monster bones. This weapon may also be able to have a block as other weapons do, but instead of a standard block it could be like just holding the saw out and letting the monster run into it; perhaps based on how much damage this does it could cause the monster to stumble or stop in place since they don't have the strength to push past your saw. I could also see a move where you swing the sawblade around and fire out the sawblades as projectiles, or maybe cause one to spin around you like a shield, but most monster hunter weapons don't strain believably to this degree. While people may accept things like the hunting horn being a giant blunt-weapon bagpipe that just gives everyone buffs from the music, people might not accept weapons that are too “magical”. I guess here I should mention the fifth weapon idea which I scrapped; a pair of maracas or shakers which act as close range dual blunt weapons that are used like clubs, but can pound out specific beats to cause rain to come in. I like the idea of the rainmakers as blunt weapons; creating healing rainclouds, dosing and curing allies of poison, or waterlogging monsters and making them slow.
 Syringe Plunger
This weapon looks like a cross between a jackhammer and a giant medical syringe. It has poking regular attacks and is a bit lackluster on the ground. The main power of this weapon is that it either has the power to launch the Hunter into the air (making it easy to mount the monster and start grappling it) OR it can like do a mini-grapple whenever it wants or is charged up. That's part one.
Then, once the user has mounted the monster (or maybe whenever the monster is stunned/reeling/asleep) they can plunge the syringe inside and suck out some of the monster's essence. In order to avoid confusion or overlap with the Insect Glaive, we'll call this resource “Ichor”. Once your syringe is filled with ichor, you can then conver it into a variety of different substances. I imagine these being sprayed out like smoke clouds. You can convert the ichor into a healing smoke, an elemental damage “flamethrower” (can also shoot water, lightning energy, ice, etc. depending on whatever “element” type the weapon is), or it can send out poison dust that hurts and slows the monster. Perhaps just pick one of these as its primary use; I like the idea of a useful support weapon that is good for debuffing the monster instead of buffing your allies in the same way the hunting horn is. I feel that much of the game's status effects against monsters are based on specific weapons and ammo types, and while cool, aren't quite as impactful as a “specialty” weapon might be.