Seems like something extremely hard to balance, since this would hard counter any scourge, well necro, symbol guardian, trap builds etc. We can MAYBE have specific anti aoe skills (ironically they would also be ground targeted aoe) that haves the efficiency (half damage and half conditions) of offensive aoes. Poison, boon rip and cover condis already counter defensive aoes. This is a very interesting idea though. Anti-aoe is needed but I don't know how anet would implement that.