@1 - from my perspective being too hard is as much a punishment as being too easy. there is a whole "science of fun" research area and it blows my mind that games have never engaged with that. with all of the "artificial intelligence" available these days it ought to be dead easy to track a players play to determine with the game is too challenging or too easy and then make constant micro adjustments to difficulty to ensure the level of challange is appropriate to the player.
The hard part is games would need to stop using the troupe that progress is measured in "more HP" and "more dps".
@2 - having played lots and lots and lots of games i'd say "consumables" are massively less fun then "cool down timers". cool down timers are very inviting to a player to use them with no real punishment while still allowing balanced gameplay. the main problem with consumables is the gameplay always is (and really ought to be) balanced for the player that does not use them. if using consumables is "required" for the gameplay difficulty then "grinding for consumables" becomes a core gameplay mechanic. i was willing to do that grind for world of warcraft but no other game has addictive enough gameplay for me to not drop the game.
3. my personal most hated paradox is how doing side quest in an RPG make the game unplayably bad. I did this to myself in horizon zero dawn right now. the game was so fun that i just wanted to do every side quest available to me (including the collection activities). but in doing those side quests i got soo much xp that i hit level 40 before hitting the halfway point of the main quest line. now as i do the main quest line the things are soo fudgy easy it is pretty much unplayable.
@1 A few games do attempt that. Most famous being Left 4 Dead.
With modern gaas it really should be all but standard. It'd be really yucky but it could be optimized off of microtransaction spending. Find the perfect amount of difficulty to push people towards buying stuff on a regular basis.
@2 I just finished Control and I loved its lack of consumables. Even guns use cooldowns instead of ammo and it just makes gameplay a lot more freeform. Every attack can be used regularly and you never "save" them for later encounters or harder enemies.
It has a lot of grindy content yet none if it is consumable focused.
@3 It wouldn't work for every game, but you could maybe have some excuse like "the bad guys are reinforcing their army more and more every day" and just scale up main quests for every completed side quest.