Abysia: Fiery Blood, Brothers of Fire, something else. I don't think Lava Land is mysterious enough.
Arcoscephale: Same for Mystic Kingdom, as strange as it may seem! I can't do better, however. Kingdom Reborn, Ways of Old, Era of Steel...
Atlantis: I *really* like Coral Towers. Short, mysterious and tells what it is about. Perfect!
C'tis: Swamp Land - a swamp, a land, so what about it? Desert of Life and Death, Kingdom of the Lizard Kings? Something Egyptish would be nice. Desert Kings/ Kings of the Desert?
Caelum: Ice Lords - too many lords, too many lands. The Magocracy, Seraphs' reich, Way of Blizzard, People of Winds
Machaka: Spider Lords is nice, but Spider Kings is better. Kings chane often, that's why I chose this over Land of the Spider King.
Man: Avalon
Marignon: Inquisition
Both could use something explaining it, but they can do without it. Witches if Avalon is the only one I came up with.
Mictlan: Blood Cult, fromt he description.
Pangea: The Beastlands, Forests of the past, Taming of the Wild
Pythium: Empire, as Illwinter writes it, or Emerald Empire to make it different from Ermor.
R'lyeh: Sunken City, from description.
T'ien Ch'i: Celestial Empire, as in description.
Ulm: Iron Kingdom, from the description.
Vanheim: Vanaheim is the home of Vanir, Midgård is the home of humans, Helheim is for the dead (IIRC). As the name Vanheim is already in use, it would need something poetric akin to the "Coral Towers". Masters of the Seas would do, but I think something better can be done.