**Full disclaimer, these suggestions are just game mechanics that I think would improve Utumno in renewed, and none of them are deeply rooted in lore. If you have lore-friendly suggestions/improvements, please share them!**
Currently in Legacy, Utumno has few challenges and elements to it. Essentially, there are just two: tons of hard mobs, and treacherous terrain generation. When you go to Utumno, all you want to do is get the drops you came for, and then get out of there alive. While this definitely works and is still an incredible challenge, I think there is a missed opportunity for something... special. This brings me to my main suggestions for an Utumno revamp in Renewed.
These structures, rather than the cave-like passages of current Utumno, would be reasons to travel throughout the dangerous area. These Structures would generate into the existing generation of Utumno, but they would add an enhanced exploration aspect, as well as a possible way to get good loot. Here are a few suggestions I thought of:
Prison Cells (all layers): A network of cells with tormented elves, or a new NPCs called "Utumno Prisoners". These cells need not be too complex (even something as simple as the cells in vanilla Strongholds), but their very presence, especially with a unique NPC, could greatly enhance player experience. These new NPCs would be passive, but run away from anything that approaches them (including the player). If you talked to them, their speechbank would likely be similar to Slaves of Nurn, or Tormented Elves. You could also get the Utumno advancement [Trapped in Darkness] upon interacting with them.
Torture Chambers (obsidian and fire layers): These would be similar to the Prison Cells (possibly even a branch off of that main structure), but with slightly different details, and possibly a loot chest with a new item, the "Utumno Branding Iron". This weapon would be repaired with orc steel, and light its targets on fire with the effect of a fire aspect I weapon (this one might be too OP, or like the balrog whip, adding chill simply wouldn't be an option). Attack stats would be similar to the Smith's Hammer.
Utumno Forge (fire layer): Here is where the orcs of Utumno forge their weapons of war. I envision a large room, similar to the large open spaces in Legacy Utumno, with vats of lava and orc forges. Here you could find Utumno Smiths (normal Utumno orcs wielding Smith's Hammers), and loot chest/armour stands with Utumno gear. If you kill an Utumno Smith, you get the advancement [Forged in Hellfire].
Failed Attempts (any layer): The last stand of past explorers. These might have a small barricade, inside full of skulls, and a chest with maggoty bread, and worn out gear. You might also find a final diary (similar to the one found by the Fellowship in Moria) with the last accounts of the attack.
Barracks (any layer): This would be the Utumno version of the Middle Earth camps. Chiseled into the walls, there could be some fur beds, barrels of orc draught, and a chest with maggoty bread, orc steel, and the chance of Utumno gear.
Spider nest (ice layer): This would be a normal large opening, except full of spider webs, they new Renewed Hanging Webs, and Webs of Ungoliant. This room would be full of ice spiders, and in a central nest (similar to the cave spider spawner in Vanilla mineshafts) there could be a chest with mysterious webs, gold, silver, rotten flesh, and manflesh.
Utumno Chests: as a brief side-thought, it would be cool if the Utumno loot was found in special Utumno Chests (similar to the stone chests, and ancient haradric chests in middle earth). These would be nothing more than a cool chest texture for Utumno.
Using new vanilla items/blocks, or adding Utumno versions: I think there are some cool new blocks that could be added to Utumno. First, the vanilla chain would look so cool incorporated into Utumno generation such as the large vertical shafts (and also in some of these structures I suggested). I also think that the soulfire items could be a cool addition, or Utumno variants of them (maybe "hellfire lanterns", etc.).
These are just a few thoughts I had on this, and please let me know any suggestions you might have more more/different structures, or changes that would make these fit better to the lore.