There's a time and there's a place for certain things
Just played a round of legendary death from above underworld and our soldier decides that he wants to get the lore achievement on legendary mode?
Like there's a time and a place my dude, and that's normal mode not legendary.
I think my absolute favorite kind of game has to be class-based co-op horde shooters. Games where you, preferably in a party of friends, mow down waves of enemies, but specifically where you've picked a particular class, so you have abilities or gadgets that others do not.
A great example of this is Deep Rock Galactic, or DRG, where the gadgets of each class compliment and work with others. For example, the Engineer has a platform gun, which can be placed on cave walls, so that the Scout, with the grappling hook, can get those previously unaccessible resources.
Another example is the Borderlands franchise, though the classes less "work together" and more "have different ways of slaughtering as many enemies as possible." In Borderlands 3, one character has helpful pets that draw attention away from the player, with skills that focus on critical hits, survivability, or upgrading your pet. Meanwhile, another character has a large, tanky mech suit, with skills that focus on high damage output, explosion damage/radius, or becoming a glass cannon.
However, as much as I love these types of games, to a certain degree, you are locked in. You choose the class which you enjoy playing the most, and are locked into that in your party for the extent of either the full story or that particular round/mission, only helping in the ways your class can.
So I have an idea. I want a fantasy, class-based, co-op, PvE game that allows you to have multiple classes.
Let's say you the maximum party size is the four, as is common in these kinds of games. Before you jump in, you each get to pick three specialties or proficiencies out of a long list, perhaps 20-30 or so. Each "class" is simple, having a few small perks, but you're free to build your character however you want. Progression works by playing with that class equipped, and your abilities in that class slowly grow stronger as you play with it and they level up.
Some classes would also naturally compliment one another, but you could mix it up for variety in case you want to do multiple things. For example, you could dedicate yourself fully to support, picking three classes which specialize in 1. party-wide healing, 2. single-target healing, and 3. offensive buffs. OR, you could mix it up, picking 1. single-target healing but also 2. bulkiness (for health and armor) and 3. self-buffing defense, to become a benevolent healing tank.
Parties could customize the skills they want to take in a particular session, working together to make something overpowered with every person playing what they want to play, without being limited by whatever the class' intended limits are, instead deciding for themselves what pros and cons they want to live with. The classes would be locked in, but there would be opportunities to swap them out at regular intervals so you weren't fully stuck. Customize, make the fantasy character of your dreams with all the features you want!
(Side note: this could also apply to a sci-fi setting as well, but I personally like the idea of a fantasy setting more).