It's really not that unlikely though. Vivienne wasn't in the survey, and there was nothing suggesting she even existed until after her official reveal.
Right, which is why I say that it's possible. However, we've gotten a lot of sources for finding companions at this point though and we haven't seen hide nor hair of another possible companion except for DHMG and Cole. As much as I'm really not interested in Cole, it's looking like these two are possibly going to the companions. Again, it's not certain. It's not even probable, I would say at this point. However, it's even less likely that another companion is out there and we haven't seen him or her in any way. Unless, that is, s/he is being kept a secret right now, which I could totally get behind.
Varric or Solas were not in the survey either.
Yeah, I wasn't using the survey as the sole source of evidence, though. There is also the gameplay footage, the concept art, the official trailers, the magazine stories, and the PAX information. The survey was just one of those sources.
When was the last time the Grey Warden was seen in game footage/art? Is it possible he was originally supposed to be in the party and they've rotated him out over the past couple of months?
I think the Warden was last seen in pre-alpha footage, so I guess it's possible. But he's actually been in several things: the gameplay footage, the concept arts, the leaked survey pictures, and the armor customization art. It seems to me like he was a pretty well-established character if he's so well integrated into the concept work. Doesn't mean he's out, though.