I like the idea!
The rewards could be something complex, or something as simple as 'you have a state? reward!'.
For the emigration, I don't think we will need more regions, nor something 'special' in the receiving islands. They will get a big bunch of nobles, probably in one big wave, with an objective: To re-create their realm in these alien lands! ... something a bit like the 'Visigoth problem' the Romans had... maybe we will see some kind of Adrianople battles!
I suppose the best way would be for the rewards to make easiest their settlement in the new lands...
mmm... Maybe the rewards could be something as simple as multiply enormously the amount of taxes received(or this and the H/P wont in battles)?
In RP it could be as a Curse... The greedy humans provoked the Gods to punish them making a endless rain of gold. Good at the beginning, but in the end, the land need water not gold! And then monsters would awake greedy too for the gold!... It's only a idea, but I would like a IG reason in this 'End of and Island'.
Maybe it's a bit strange, but I hope to have a character in the sunk island!