IN UPDATE 3.1.0:

NEW Island - the majestic Magical Sanctum
NEW Magical Monsters - Xyster, Knucklehead, Frondley, Déja-Jin and Larvaluss
NEW Magical Monsters coming every 2 weeks
Single-Element Monsters added to Market on Ethereal Island
Ability to send Keys to Friends directly from the Friends Menu
Streamlined Battle Tutorial on the Colossingum
NEW Versus and Champions' Guild battle modes - win competitive Matches to improve your Rank and win great prizes
NEW Friendly battle mode - face off against your Friends just for fun
NEW Jukebox feature in the Mess Hall - unlock new tunes
NEW Decorations exclusive to the Colossingum
NEW Avatar feature - choose a Monster portrait to represent you in the Options Menu
Costumes available to buy on other Islands in the Monster World
StarShop added to Shugabush and Ethereal Islands
Current and previous weeks' Versus Mode Champions in the Rank menu on the Colossingum
Improved interface for secondary Currencies on Island display
Ability to Teleport Monsters to the Colossingum from more Islands