I can see why people may want something more interesting since there are times that I started to fall asleep during dungeon runs (which is my own fault since im going for all jobs to 80 so dungeon fatigue is a thing) but I am rather happy with how dungeons are laid out. Sure Satasha and Toto-Rak and other ARR dungeons got side rooms but they were almost never bothered with cause people wanted to run through them as quickly as possible unless the entire group was new.
The people who say they want large massive dungeons like WoW. . .I will borrow a quote I keep seeing pop up on their forums (paraphrasing) 'You think you do but you don't'. Having went back to Legion after being given a week free of game time (and I think even Legion cause I guess they wanted people to come back), the dungeons were pretty but boring as hell in terms of navigating it (I honestly don't remember a lot of the bosses). They were huge with lots of side rooms, mobs, and areas to explore but guess what? No one cared.
Want to go explore an area that has nothing to do with getting to the boss or has extra mobs? Better do it on ya own or with people you are in a guild with because the moment one extra mob is pulled the rest of the group will explode on you for pulling more mobs than needed and not skipping them to get to the boss. As often as duty finder is used I can see the same thing happening if FF14 started to add these same sort of dungeons. People would find the shortest path to take, skip what mobs they can, and if anymore are pulled that means more time spent in the dungeon rather than being done and I would be one of the few people if a vote kick came up to remove someone who was pulling the rest of the mobs on purpose, I would vote to remove them.
If wanted to add bosses like the second boss of Mettle (someone mentioned that earlier) than cool. I thought it was fun. Not having to fight but do a puzzle of sorts is ok with me. Or some other suggestions such as random mini-bosses for extra drops or hell, give a roulette that gives a random set up of the dungeon with random bosses already added in the game and random trash mobs in between. Something to mix it up is fine but adding large dungeons with extra rooms and more mobs in akin to something like WoW? Again, you think you do but you don't.