Here me out. I know there's going to be some push back on this because it would pretty much take the amount of work & time a whole X-Pack takes, but I feel like it would add a huge amount of replayability to the game and give it something that no other modern MMO has. (Which means $$$)
With ESO's player housing system, you're able to place objects & creatures in such a precise way that you can actually give your NPCs actual pathing that you then edit down to even the time at which they stay at each node. This got me thinking that a dungeon creation system could also use this exact same system that already exists in the game. Below you can find a very rough outline of what a system like this would look like.
1. Pick your theme
Your base race would give you that racial dungeon theme to work with. If you're an Argonian, you can work with something that looks ripped straight out of Murkmire, if you're a Nord you get a Draugr crypt/tomb, etc. This means that the base racial dungeon styles would use assets that already exist in-game and would only need to be repurposed for the new system. More dungeon styles could be unlocked in the same way we have crafting motifs.
2. Map your dungeon
After you have your theme selected, you build the dungeon itself. Each dungeon would start with a room designated as "Entrance" and you would build "Pathways" that connect to "Rooms" branching from it. "Pathways" would basically be your hallways and tunnels that connect each main "Room" of your dungeon.
Think of it looking a bit like creating your own DOOM map. Which, fun fact, Zenimax has already had some experience with thanks to DOOM 2016's Snapmap system.
3. Place objects/enemies
This is where the existing player housing system takes over. The only difference is that instead of placing furniture and house guests/pets that you own, you're placing objects from your dungeon's theme and using up a points (I'm picturing something extremely similar to Halo's Forge system.)
You would add pathing for the enemies, lay traps, and even place special "Enchantments" in each room that would alter combat in some way while fighting enemies there. Maybe one room has a "No blocking" rule that causes the player to take a small amount of damage each time they block an attack. Maybe another room as a "No healing" enchantment.
This would also allow for player created "Stealth dungeons" where each room has a "Don't be detected" enchantment.
4. "Upload" your dungeon
You know that custom game browser everyone wants in the Master Chief Collection? Add it to ESO and allow for a ratings system so the top-rated dungeons are easy to find and play. I don't even think a reward system would need to be tied to it. Having created a dungeon that thousands of players have played through would be an achievement alone.
What do you think? Would it be worth it? Would you enjoy making your own dungeon? Would you enjoy playing other player created dungeons?