For me, Monster Hunter started to lose its luster back in World. It was a great step forward with many of the things it introduced. A seamless world, improved visuals, a more fluid combat system, "tracking a monster" was a cool concept, various small additions like the slinger, etc. For once we had a proper tutorial and a way to ease a player in with a story instead of a block of text on your first quest with a Sword and shield that you know nothing about.
However for me what it lost was the quick get in-get out grindy nature the series was all about. While that can be argued that its better to not waste a dozen hours to get a single mantle/plate from a monster. The alternative of getting everything handed to you has severely diminished the rewarding sense you get after toppling your 15th Silver rath for a ruby or plate.
Along side that changes to the structure of the game has given it a more on rails feeling, especially Wilds with its 10 hour tutorial. Guild hall has been reduced to literally nothing. Use to be a split between story quests, and the "Real progression" of Monster Hunter with a difficulty bump. Gone are key quests, and a quest book of things to hunt. Instead I'm drip fed exactly what I need to progress in ranks, with very little deviation to fight monsters I don't NEED to fight, but absolutely can, instead I can just repeat the ones I already did. Its not until late game when the game opens up with a variety of random quests, and a "tempered" system to force this weird arbitrary "endgame GaaS" feeling grind to min-max my extremely limiting builds.
Rise came along and shifted things around and tried new things, for better or for worse. It also refined much of what world did in my opinion. We had large open maps in World that ultimately were quite expansive for the like 2-3 zones you'll be in during a fight, and well over half of them aren't used. Rise refined this, shrinking down the maps, introduced a pretty good movement system (Including the wirebug for recovery and launching yourself around), and mounts. The guild hall also had its proper return with 2 separate quest lines again. It felt like a merge of Classic Hunter and Modern Hunter. It didn't do everything right, but it was a cool entry none the less.
Much of Rise felt like small improvements of systems they made in the previous game. Which means Wilds should take these and run.
Instead we have probably the most unique entry in the series. They ran with the idea of large open spaces and introduced more to make all the spaces more useful and interesting. Combat is further refined. However we're playing on rails for 10 hours before we can actually get to the major part of the game everyone wants. As such after you hit end game, it's just you grinding the same 3 monsters over and over again for super small Min-Max upgrades. Endgame weapons are basically all just artian gear. Gems being limited to "Weapon and armor" only gems makes the already watered down skill system of the classic games, even more watered down and less interesting.
Now when it comes to these games longevities these games since World has been a focus on making the "forever game". Pushing more arbitrary grinding for small mirco-numbered, min-max improvements instead of pushing for optimizing your gear and builds (Which has tanked in variety and depth severely since the prior games). The fan base as also built a weird Meta around the series as due to the nature. These never feel like they belong in Monster Hunter. Its a series about unique builds and fighting big monsters to make gear from them. Rinse, repeat. Once you basically killed everything the games are done. Maybe you'll go after long term goals such as unlocking monsters (A system also abandoned in the newer games), but otherwise once you've done it all, the game is done. No need for this "For ever game" mentality.
My perfect Monster hunter would be to have the entire roster from the past games available. Bring in Frontier monsters too. Bring the old quest system back, using the Wild's open world. Bring back the old skill and decoration systems (I want gems that give you negative stats again. Made builds way more interesting). New combat system is fine, but bump up monster's health and damage they deal. I want things to hurt and force me to optimize my playstyle. Stop handing out everything to the player on a silver platter. I want my rewards to feel like rewards. Not handed out.