Prologue
Remember the golden age of RPGS back on the NES? (of course you don't, you are all so young :)

The original Final Fantasy was one of the most influential RPGs of all times. OOO has a history of alluding to various things from old games to other genres. It was a standard RPG trope, in my mind established by Final Fantasy, but i could be wrong, to have the game based on the pillars of sub-bosses before the eventual build up to the main boss; FF set the standard of making you fight your way through elemental based themes. In the original Final Fantasy, this took the form of the four fiends, EARTH, WIND, WATER, and FIRE.

I'd like to humbly propose an idea inspired from, nay stolen, from this. Of course, to serve SK's purpose, we shall reconstitute this around the themes found in SK.

The Goal
The idea here is to create more Tier 3.5 level stages--if properly equipped--but to up the ante by making them extra dangerous if improperly equipped. This is to encourage the production of more various armor types, the usage of a wide array of trinkets, and/or the procurement of Unique Variants. I won't outline the exact particulars, just the general idea. It would be up to OOO to balance as necessary, and i'd let there creative staff handle the creative creation of bosses (if any). I'd recommend that the monster families that populate these stages be randomly generated. That way, you'd be just as likely to fight primarily beasts in the shock theme as you would jellies or fiends. Again, this promotes (for optimal defense) the usage of all sorts of different armors. You'd be doing well to have Merc Quicksilver set for shock wolves, and a max shock skelly set for shock fiends. Vog would work well for fire undead, but you'd not want something else when the stage had fire + jelly. You get the idea.

The Idea
The idea here, is to make new dungeons, with "Fiend" mini bosses at the end. The noun "Fiend" can be replaced by something more in-line with teh steampunk nature of the game. But the idea remains the same. As Final Fantasy had the four elemental bosses, so to would these Fiend lairs be built around our "elements."

Shock, Fire, Freeze, Poison, Stun, and Curse.

The general idea is as follows: Each themed expedition takes you to an environment where the world is made up of that status. If you were to enter that theme without that protection, you'd be constantly under the influence of that status. So basically, the entire level is a giant status zone. It would be a lower infliction of the status, thus allowing you to gain immunity via armors and/or variants and trinkets. So if you didn't have poison immunity, you'd spend the entirety of the run poisoned. If you didn't have shock immunity, you'd be in a constant state of shocked. (the Freeze would have to be adjusted to allow for a cooldown affect, so you could still progress while getting froze constantly).

Upon reaching the end of the journey, you would face a mini-boss, much as we've seen in the danger missions. The payout could be a semi-rare mat, or a treasure vault, or whatever, doesn't really matter. This is more of a focus on forcing players to gather and equip a myriad of armors.

Note: i'd suggest making the status weak enough so that immunity could be gained with two armors of that status type + 1 or 2 trinkets. This way, no one could argue it was merely pay to win. It would be accessible to anyone, and would encourage players to collect and equip more armors. It would also create a new goal to work towards for newer and some older players as well [the process of making new armors so you can proceed to these dungeons].

Ultimate Fiend Challenge
As an alternative or Add on, the ultimate challenge could be a dungeon that contained the same idea, but with all 6 status affects (one for each floor). this would require the knight to possess the required equipment for each theme in order to safely progress to the bottom. It would be a new endgame challenge. Something to force endgame players to have an armor set for every possibility. --This of course would have arsenal stations each floor. Or, if 6 floors is too much, make an arsenal station at the midway point of each stage, and make each stage have two distinctly themed halves.

What the game NEEDS
Some sort of end game content that focuses on rewarding the player for being prepared (defensively) for that specific situation, and likewise harshly punishes for coming in ill-prepared. End game dungeons with some fashion of an invisible "barrier to entry," if you will, but make that barrier something everyone can attain. (not purchase only DLC, or shadow keys).

I'm not saying OOO needs to use my idea
This idea was largely spitballed, made up as i went. I'm not saying, OMG this is the best idea ever, and OOO would be foolish not to use it. I do feel that the game desperately needs ^see above^ and this is just one of many ways they could attain that goal. This idea is merely that, an Idea. It would be fun and challenging and interesting. But generally, everything OOO implements is Fun and Interesting (if not always challenging). So this idea is just to create discussion and get people thinking. I'm sure there would be better ways, just as fun, challenging, and interesting, to reach the goal of what i think the game needs. I'm sure anything OOO did in that vein would be awesome, and have tons of replay value, and would give us all, new players and veterans alike, incentive to make and use otherwise underutilized and worthless armors. It would be end game content that had more accessibility than shadow lairs, and more life and greater variety than endless FSC runs.