For starters, don't design it for level 17s who have never played the game. Don't design for four DPS. Design it for a standard group comp. If four DPS can get by with pomanders - good for them! It shouldn't be the standard though since it absolutely makes for boring content like FATEs. It is 100% ridiculous that at this point in the game's life we're STILL getting content for people who haven't played in months. The leveling dungeons in this game try to teach you mechanics and good party play, but at max level you throw that all away in favor of monsters that do nothing but throw out telegraphed, easily dodged attacks. See: original incarnation of The Vault vs any """"Expert"""" dungeon.
Add more unavoidable attacks to monsters. I'd basically remove all telegraph zones and cast times, barring a few like Self Destruct and minotaur/cyclops 1k tonze swings. The big orange circles you can easily step out of are part of what makes this place so stupidly easy. I'm not asking for Savage difficulty, I just want the dev team to stop treating us like idiots who don't know how to work a mouse and keyboard.
Second, add events on floors. Not every floor needs a special event/goal, but at least one per instance would be nice. I'm cross posting this from another thread, but basically steal ideas from good games. Here are some I thought of in five minutes:
Oh no, a Tonberry is chasing you (this one is from your good game, ARR around 2.0) so you have to move quickly through this floor.
A mysterious merchant has appeared who will let you trade 2 pomanders (stop it loc team) for one.
Certain monsters on a floor have had their stats turned way up. They have a black aura.
Cultists are summoning a boss on this floor. Get to them and kill them before they succeed.
You found a magic lamp with a genie inside who will grant you a wish. Choose wisely (+weapon, +armor, +exp, +gil, or a massive buff for the next floor)
This floor has no map and is shrouded in darkness. Stick together and make your way to the exit.
This floor's exit is behind a caved in wall. You need to lure bombs to the wall to destroy it.
A treasure goblin appeared. Where have I seen this one before?
There are 4 switches that you need to stand on at the same time. Split up and go do it.
A miniboss appeared. Kill it.
Your whole party is transformed into monsters and you have to fight "adventurer" NPCs this floor.
This floor is a tower defense mini game.
On this floor you're fighting Ifrit.
Your party has been split up and you need to get back to each other to advance.
Every room has a door that you need to answer a riddle to get through. If you fail a riddle you fight a hard monster.
Add "champion" type monsters at random with affixes like in Diablo. "Screeching Icy Puk Champion" that casts AOE silence (screeching) and leaves a trail of ice behind when moving (icy). "Super-Heavyweight Exploding Gargoyle Champion" who moves very slowly but hits super hard (Super-Heavyweight) and explodes when he dies.
These guys could drop bonus stuff maybe, which leads me to...
Use this place to experiment with new stats and effects. Balance does not matter here at all and it's high time for some new stats. Speed/Det/Crit are played out. Add some interesting stuff to Aetherpool gear at random. Oh snap, while going through the dungeon this time I got an Aetherpool enchantment that restores HP when I attack! Oh yeah well I got one that reduces my cooldown timers, etc.
Really, just take some risks and do something creative. Not every piece of content has to be beatable by a newbie on their first run.