Double damage weekend - all your spells and attacks deal double damage to all monsters. Monsters deal normal damage.
Chimera of intensity weekend - all players are granted this boon for the weekend. http://gw.gamewikis.org/wiki/Chimera_of_Intensity
Extra regen weekend - all players gain +1 health and +1 energy for the weekend. (this might be interesting

No death penalty weekend. "LFG, we don't need monks sorry!"
Reenforcements - Two extra henchmen/players in a team (some places would need extra henchmen added temporarily) "Finally, we can do THK!!1".
Double elite weekend - players are allowed two elites on their skillbar but on the same attribute.
Ten skills weekend - players can have ten skills on their bar.
Double health weekend - all players have double base health.
Double energy weekend - all players have double base energy.
400 attribute points weekend - never before has a players secondary been something to be wary of!
Return of the wreckages - all the old wreckages are returned temporarily. Ah the memories!
Droknars danger zone - Droknars forge is a pvp free-for-all and the NPC's are not amused at the anarchy. They fight anyone who comes near! "who spiked the dwarven ale?!".
Tweak as required but i think there's some interesting things here. Would definitely throw up some interesting (and probably near invincible) builds but at least it's only temporary and all in the name of fun.