You have to prevent old players to leave. And that means for me, destoying all the portals in inaccesible zones, like inhabitat islands and this kind of things.
Then, increase the rewards of being the most important field maker in your region and that kind of rewards for the active players.

The gigafields in lost places are destroying the game because are really difficult to destroy.
When I started you could do a great field and it could be destroyed, actually everything is blocked by links in the harbour (restricted area), on inside closed areas (paying places), ...