i've been playing since 2.0 and it still amazes me how all level 50/60 dungeons are about the same. They're alike in a sense of you kill trash, bosses and that's it. Dungeon completed.
Imagine having actual dungeons with puzzles, objectives you have to clear before you're able to go on to the next boss. Like you could scatter notes containing a code randomly in the dungeon, having to enter the code to open a gate. This would not only be a change of the usual trash, boss, trash, boss thing we're seeing for years now, but it would also encourage people to actually cooperate with eachother. Right now when i enter duty finder the only words that are spoken are hi, bye and thanks. In order to compensate time spent in the dungeon you could put less trash mobs inside, this assuming the average player still wants their dungeons done in a somewhat quick phase. Puzzles are definitely something that should be easily implementable with some thought.
Imagine being in a huge room with your party, having no mini map and it's pitch black, there's only this tiny field of view around your character. You can't see monsters unless they're inside your field of view. In order to get to the boss you'd have to find embers scattered in the dark (RNG locations) to light up the room and open the gate. Simple objectives could make a dungeon a lot more fun.
To make it more clear what i'm talking about, imagine this kind of view in the dungeon:
Find the embers and return to the starting point, to light the torch with the embers.
To counter the linear dungeons you could even make a labyrinth-like dungeon. For example you could spawn somewhere random in the dungeon and take different paths, searching your way towards the final boss, no minimap. Everytime you enter a room you have to kill whatever's in there in order to advance. You could even make a prerequisite for it. Like having to search and kill mini bosses in the labyrinth before you're able to challenge the final boss. Instead of normal trash you could even use stuff like the last boss of stone vigil HM. put like 3 of those in a room dealing quite some damage and people will actually have to react to trash. You could make the party spawning location in the labyrinth rng, or you could make the boss rooms to be in different places everytime. This way the dungeon would be interesting to do considering it's different every time you'd do it.
Then for bosses i feel like there's a lot of room for improvement as well. Why not split up the party at a boss for instance? Bosses could be a lot more interesting. Imagine a two floor boss fight with 3 people fighting the actual boss and 1 person's on the other floor. Depending on the situation on the floor with the actual boss, the person upstairs would have to react to it. Like having to drop weights on a boss from the 2nd floor in order to be able to damage it. You could also make this more dynamic. Imagine the person on the 2nd floor having to press buttons whenever something turns up on the 1st floor. Like electricity appearing on the floor, person upstairs has to press a blue button, fire starts covering the floor, person upstairs presses the red button etc. Ofcourse you can't be pressing buttons all fight long so you could even add stuff to protect on the second floor.
So there you have it, there's a lot that could make dungeons or even raids more enjoyable. Not only you'd have dungeons that aren't the same everytime, but you'd actually have to communicate with people instead of entering and leaving without a word said. This is a MMORPG, but sometimes i feel like i'm playing a single player game. A simple change from the usual thing could make a huge difference.
TLDR; Simple things as adding puzzles, objectives, anything that makes a dungeon less linear or even separating the party at a boss could make the dungeon/raiding experience a lot more fun. This would also encourage jolly cooperation with your fellow players.
EDIT: Note that these are just examples. There's a lot of things out there that could make dungeons more interesting, even simple stuff.
Even sastasha NM/The sunken temple of Qarn NM are more original in this way than ANY level 50/60 dungeon. Sastasha having a note with a "riddle" on it. Qarn having the little stones and having to kill the statues on the platforms.