Hm, how bout people can donate resources to specific bases to upgrade them? The upgrades could be specific modules or upgrades in like base tiers (tier 1, tier 2 etc.). When a base is lost it loses its upgrades, which adds incentive to invest in particular bases and hold them. The higher tier costs could be astronomical, making the max tier a base could achieve a serious empire effort. It could have very significant benefits, and therefore that would be more incentive for other empires to make coordinated attacks on those strongpoints, while holding their own. Lets just say there could be only a limited number of certain tiers per continent.
It could even be that high ranking commanders will be the ones to coordinate all of this. Hmmm metagame, strategy, me gusta
I think some of this has been discussed here as well:
http://www.planetside-universe.com/s...ad.php?t=40463