I'll try and keep this one short and sweet due to word count limits.
Playing FFXIV since ARR Beta2 I have seen so many cool and interesting dungeons that come with each patch, designs that are incredible to traverse through, and places that stay within my memories for years after, such as the times I went through Coils 6 (2nd coil part 1) and a lot of 50+ dungeons from ARR.

And even though I adore the boss fights and creature designs, I have noticed a trend in the design of dungeons since around Stormblood? maybe even earlier? (and you can see it in a lot of dungeons) of the typical way, the party progresses through the dungeon with the typical 2-4 groups of mobs and pulling them "wall to wall". Going through the dungeons in the past expansion and what we currently have, has left me yearning for more approaches to how we handle going through content, even to something as simple as a direction change. I felt like before we had a bit more verticality in the dungeon pathing, in comparison to the past year of dungeon content.

What would I recommend? I'm not going to say "this is a perfect idea and must be followed through verbatim!" but something I would be interested in seeing is different non-combat mechanics to tackle dungeons (nothing too complex) or maybe even a variety in the number of people who go into an instanced dungeon. For example, one dungeon can have 8 party members, maybe in 2 separate parties within the instance, who have to start off together and eventually split off to tackle separate halves of a dungeon.
Maybe even have a strange 6 man dungeon (with a static amount of how many of each role is needed) and maybe have them take on even bigger baddies!
A lot of what I would consider interesting to see in dungeon content comes from my time doing the ARR coils and HW raid fights (both normal and savage).

My final thoughts are, that I would rather see new and interesting ways of spicing up the way we run dungeons, even if every once in a while we get a stinker, rather than having a safe repeated method of doing the content.