Since there is an expected PvP update, might as well add arenas for Khrysalis, Polaris, Mirage, and Empyrea. Adding arenas for the Tale of Four dungeons would be nice as well. I love seeing new scenery, it's one of the things I most look forward to in a new world. Maybe add different themed areas per world?
I saw a suggestion from a poster of November 29th, 2015:
Wizard City: Hanted Cave arena, Cyclops Lane by the water arena, Crab Alley arena.
Krok: Outdoor Oasis arena, Tomb of Storms arena.
Marleybone: Outside on the streets arena, main area arena.
Mooshu: Water dojo Cave of Solitude arena, Kishibe Village arena.
Dragonspyre: The Crystal Grove arena, Past Dragonspyre arena.