I would make the players have to use a candle to burn all kinds of random bushes in order to find the entrance!

Serious points if you get that reference, but there are definitely some elements from coils and deep dungeon that I would put in there. I like the idea of the group having to split up, random rooms, respectable mini bosses with soft enrages, and bonuses for the group doing certain things correctly.

Mini mechanics are pretty cool too. I liked that the new dungeon had trash mobs place an AoE marker on a player that has to be taken away from the group. I would put some other interesting things in there, like being able to lure a mob tethered to you into a trap to eliminate it quickly and also get a lot of exp from doing so.

I would also put devices you can interact with (this more or less has healers in mind) that can be used to damage the enemy or cause status effects to them. Mechanized crawlers, turrets, crossbows, awakening golems, etc. Kind of like Castrum. Just something for them to do if no healing is needed. They would be a one-time use though to not divert focus to much.

Music is something I would mess around with a lot too. I would try to incorporate different tracks into it to add more depth to the instance. If for example you were in Dohn Mheg and the music starts off lighthearted and upbeat, but after beating a boss the music becomes darker, and more eerie as you progress further into the dungeon.

In the case of things like dungeons and housing, I really like the art of illusion, and making things seem much bigger than they actually are. My dungeons would still be linear, but they wouldn't feel like it.