In the world of dreams, anything can happen.

Literal

Dream [possessive-の] world [at-では] any [possessive-の] thing [even-でも] can-happen.

起こりうる combines 起こる ('to occur') with 得る ('can') — 'can occur.' More formal than 起こられる or 起こることができる. A generic statement about the possibility-rich nature of dreams.