100% agree with you OP, I've been telling my friend all the time about the dungeons that they all look great but the designs are terrible. Just too linear, no choices, no imagination, no replayability and just the same boring formula.
While ARR probably has the best designed dungeons and there are a few good dungeons here and there that try something different there isn't enough and they don't try to think outside the box too much in making some really good quality dungeons.
1. in some dungeon you should always get a choice to go this way or that way and sometimes even have many tunnels to pick from giving lots of choices, there shouldn't just be one linear route in getting to the end, there should be dead ends, locked doors, optional rooms or different routes to take which are all different but all lead to the same or different exit. Each route could also have it's own boss that is different to the other bosses.
2. in a dungeon you ain't suppose to have a map, you ain't suppose to know which way to go... in ff14 having a map takes away of any sort of exploration or getting lost and trying to find the way out, even though if you didn't have a map you wouldn't get lost anyways but I mean this as if there was more additional areas to explore with more routes to choose from etc.
3. RNG there needs to be more randomness in dungeons not always fighting the exact same monsters or knowing where all the chests are there should be hidden in different locations. Maybe there are additional hazards to avoid like traps or depending on time of day different monsters appear or different things happen like if its nighttime or a different weather etc
4. give more of a reason to go back into a dungeon, I find most of the time when you finish a dungeon for the first time you have little to no reason to go back into it only if you get it in roulette again or levelling another job but there should be some kinda exclusives that make it worth going back, maybe some dungeons are tied to some side quests so you have to redo them again and complete some optional task example a hidden boss due to having a key item to make them appear or having to help guide a npc through the dungeon and make sure they survive during the bosses etc in order to complete the quest.
5. add harder optional dungeons, while the MSQ and levelling dungeons should be easy to finish there should be some challenging dungeons where you don't always expect to win as maybe the bosses are very hard or maybe you can run out of time due to getting lost in trying to find the way out because the dungeon might be very complex like a labyrinth etc you could probably add in some puzzles to figure out as well adding more thinking about what to do.
The current dungeon formula is extremely lacklustre and needs some proper re-imagining I feel adding a few additional things that I have mentioned here would really make doing dungeons more fun and refreshing. As it stands the current dungeon formula makes them throw away dungeons with no sense of coming away from one after finishing it and thinking it was incredible experience.