In terms of an Open world raid Dungeon in the veins of Sphospae in the level ranges of 20-40. The other dungeons in the game that have been there since the beginning do not feature the same type of focus on combat. They are fun to explore though, but be warned of the polarizing difference in the mob levels in there. they feature NM's but no chests or end bosses.
your level range open world dungeons
Nanawa Mines
Cassiopea Hallows
Muytuy cellars (forgot how to spell this one exactly)
35-50 level range open world dungeon.
Copperbell Mines
Mistbeard cove
Something something (for gridania, below bentbranch)