ArenaNet has been holding on to Bubbles for around 7 years now and we still don't know anything about it, so I think that when they do introduce it, there should be no warning, it's just sudden and shocking, and I believe the perfect way to do that, is with the death of Jormag. As we know already, the Icebrood Saga is going to be dealing with the ice dragon, I think the best way to set up the DSD's appearance is for good ol' Bubbles to deal the killing blow. Here's how I imagine it, it's the final battle against Jormag, we've rallied our forces and are hitting it with everything we've got, but it's not enough. We may have injured him yes. But at the cost of too many people, too many friends, but then, right as Jormag is going to add us to his Icebrood army, you hear a huge rumble, coming from something so big even Jormag stops to wonder. Then out of nowhere the ice shatters, and some eldritch horror, the likes of which we've never seen appears, and kills Jormag, absorbing all of his magic, and then retreats to the depths once again. This would leave the Pact and it's allies shocked, horrified, and confused, because a) The DSD came from nowhere, b) Elder Dragons almost always avoid one another, and the most terrifying reason, would be that Bubbles just killed another ED, without even breaking a sweat, which is a terrible thought. But I think a reveal like this fits with the theme of the ocean, in the depths, you never know what you're going to find, the creatures down below are beyond imagination. The ocean strikes quickly, and without remorse, and most of all, the ocean depths scare the absolute HELL out of every living being on both Earth, and Tyria, what do you guys think?