I wouldn't like to see Daimons or other Beluaterra-like invasions come to other islands. This creates a PvE game. It can be fun, but pure PvP is fun too. If you have invasions that "test" other islands, basically that means forcing player-led realms to band together against the invaders instead of having wars between themselves.
There are ways to reduce the land area of other islands without having an invasion. The auto-blight idea for rogue regions surrounded by other rogue regions may work, and doesn't need an invasion.
