the zombie virus is natural, not man made.As opposed to the cliche "evil scientists creates zombies" idea, the virus in Dead Island is simply a natural mutation of the (real life) virus Kuru, which was caused by cannibalism practiced by the local tribes
The zombies in the game are not actually "undead."This explains why they take damage from being underwater, and why higher-level zombies (who would naturally be exerting themselves more) take drowning damage faster. People don't actually die and then reanimate, they just go into a temporary coma while the disease takes effect.
In Dead World, Dead Island 2, or whatever they call the proper sequel, instead of playing as 4 new protagonists, we'll play as mutant versions of the original 4 protagonists. With its heavy Borderlands gameplay inspiration the series is already pretty silly and outrageous. Instead of doing yet another zombie survivor FPS game in a market that's already saturated with them to the point of parody, Techland could be original, take the franchise in an unexpected direction, and have us play as superpowered zombies. Alternatively, the protagonists could at least be given a Super-Powered Evil Side (like in The Suffering) to replace the Fury mode.
The next game, Dead World or whatever they call it, will take place actually on Australia.