First of all, thanks se for this old school idea, i love it.
I don't know what you have in mind with this so i'll just day dream a bit and call that feedback. Hopefully more players will share their ideas, but for now these is what i got:
Please no proc system or atma/atmacite, keep it simple and difficult like dungeon crawler should be, no temps, no time limit, 6 players (only 6 nothing more nothing less) and a lot of floors to explore (maybe with continue option every 20 or something, like nyzul, but even without it's fine). Fill the dungeons with: trap door (like the ones in castle oztroja), door and chest locked that need key dropped from monsters or NM to be opened, mimic, fake walls that leads to secret area, large rooms with a lot of monsters, labyrinthic tunnels with roaming nm, ??? that gives hint on what to do to advance to the next floor or found treasures etc.
I don't know if can be done with the game engine, but having a map that reveals itselfs while you navigate the enviroment should be good, if not possible just no map will work fine. I hope this will be something really really hard even if you have empyrean/relic/mythic weapons and other great gears, so everything must be well thought, with good use for foods, medicines and strategy. If even regular monsters can be a challenge maybe this will lead into choose what worth to kill and what it's better to avoid (or try to avoid) and kill later. I also hope that any jobs can be useful if the party is assembled properly (damage type resistance or immunity can help a bit).
The most important thing is to have very good loots on this, i don't care that much but most people do and i don't wanna see another Moblin Maze Mongers (great event, lot of fun, but few people to play), also if floor advancement/rewards/difficulty are well balanced there will be fun for all kind of players, and best rewards for the most skilled ones.
I hope that could help, thanks for reading and sorry for the poor english (not my first language).