Europa or Mercury. If it is Mercury, I think they'd have a good opportunity to add in the burning mechanic they already have in the game into that raid. Could be cool, plus, vex was a cool concept in the first raid, I wouldn't mind doing it again. As for a different element, I feel like poison would be pretty cool. I mean, the mechanic is already there with the Thorn and smokes. All we need is the actual damage type.