Not quite the same as a full blown dungeon with a boss objective or loot objective, but there are level 30ish locations that could be explored for giggles.

Nanawa Mines north of Thanalan's Camp Black Brush.
Muay Thai Cellar north of the Black Shroud's Camp Emerald Moss.
Cassiopeia Hallow south of La Noscea's Camp Bloodshore.
With notorious monsters in each that may have some loot of interest, count toward an achievement, and prove a difficult encounter if you're not a higher level or in party.

As you level up to 40+, you might investigate locations such as Copperbell Mines southeast of Camp Horizon, Mistbeard Cove north of Camp Bald Knoll, and the Lara Croft dungeon south of Camp Bentbranch. Couldn't tell you what are in these except for giant ant lions and diremites that make me sick in Copperbell.

The beastmen strongholds at level 48+ may also prove an interesting venture to see how far you can get solo. There are some enemies that drop keys to chests that contain loot. You may find some competition for these items.