All Planetside needed to keep me from jumping in for a month and then out for 3 months was new base maps. I wouldn't have cared if I had to download everything again. Bandwidth access was not a problem a few years after launch.
Whenever I think about what would make Planetside more sticky for me, I start thinking about DIKU grind mechanics. And I feel ashamed.
I think maybe more low-impact activities, like ANT runs, only 'better and more varied', and a game dynamic that generates all kinds of battles, from small tower skirmishes, to big continent offensives to some kind of uber achievement where an army 'wins' the war for a short period of time before resetting so we can get back to it.
Yes, I'd proably crib a few more ideas from CRPGs instead of just levels and xp. Keep it an FPS, keep the massiveness, but allow players to do more and accomplish more than respawn and wade back into the zerg.
"What I'm saying is you should make friends with a few catasses, they smell funny but they're very helpful." -Calantus makes the best of a smelly situation.