Goes along the same line as the recent 2x XP and 2x Damage events...
Pick a non-end game, non-set item pink and make it ridiculously (to an extent) over-powered. Maybe even with some crazy proc(s)!
Perhaps said item can even give an xp bonus!
I'm thinking the actually OP-ness of the item(s) should scale depending on what campaign the user is participating in, or perhaps just scales to the Level of the user. I'm not sure how difficult that would be to implement.
It's gotta be something that's still fairly tricky to farm (Plasma Pyramid pinks are out, etc). Balefort Pinks are good to allow lower level players to participate.
Probably a good idea to have one of each (dex/int/str) family be part of the event.