So a friend and I were chatting about the event and world bosses and we came up with this idea:

Make zone bosses spawn more often, perhaps independently of player numbers in the zone, like the world invasion, but more than daily (a despawn timer could be implemented after the boss is killed);
Players to fight over wardstones and rifts completed in a zone;
The faction with the higher percentage (or threshold amount) of completed rifts and wardstones protected locks the other side out from obtaining the best sourceshards from bosses (they can still get some conciliatory reward for participating).
The boss should be immune to damage until one faction has control (i.e. awaiting to be summoned by minions, or wandering around the zone like Alsbeth did).

It makes more sense to have the zone bosses spawn first so that players have an visual incentive to actually work towards/fight over. Currently, this s what happens on my server:
/5) [Playerblingbling] Boss up in shimmersand
/5) [Bhutasalt] yay, finally...

And then upon arriving in shimmersand, the boss is half-dead and the only thing that matters is dpsing enough so that the boss loot table recognizes my contribution. Or sometimes the zone event hasn't even started.

FWIW, I have only ever received one purple sourcestone and that was in silverwood during the first couple of days of headstart, which was also completely by accident. I had no idea wardstones and world bosses even existed. There was just a big tree that I got to hit with a sword. Since then most people--at least on my server--haven't seen a purple sourcestone since, nor do they have any idea how to get them.