For real? Oh man, I can't see that sitting well for some people.
Not to mention, the writers seem to really enjoy torturing Hawke, given what happens to his family. Now murder every (surviving) companion on top of it? It really would drive the "Hawke is helpless" theme home. 
I don't know about 'every' surviving companion. I don't even, technically, know that they were planning on killing off any of them, just that Gaider's wording implied it, and he was being quite vague.
Unfortunately, now that his Tumblr has been taken down, I can't find the source.
FOUND IT
It was indeed a difficult progress to let go of the DA2 expansion’s story. I mean, we never got very far past the planning stage, so I’m well aware that by the time it got to release it would undoubtedly have changed significantly — perhaps even to my great disappointment. Even so, it’s still hard to avoid thinking of what-might-have-been.
So you’ll have to forgive me if I say I never intend to go into detail on it. For my sake as well as the sake of fans, many of whom lack my perspective on such projects — if it’s hard for me, what might it be like for them?
Though they should be happy, to be perfectly frank. The ending we had planned for the expansion was brutal, insofar as some of their favorite characters are concerned…which would have been delightful for us writers, but maybe not so much for them. Maybe it’s better this way.
Or not. Whatever the case, I’ll just say that some basic plots were carried over, and naturally changed as the context for them changed. The Temple of Mythal is a good example. Beyond that, I’ll leave it to your imagination.
In the main game, this worked, but in Awakening, it became a bit of a stretch. I mean, a Dalish or City Elf running the Arling of Amaranthine? Fetch my qamek, Seamus! I never asked for this.
Easily the best part of the expansion.