Reading through the comments, I'll have to add that Cassandra is by far the most likely companion that sticks with you. She's been advertised in every trailer, probably the first companion announced, has a movie dedicated to her...I doubt the writers would want the option of having her killed off in Inquisition, probably because she'll appear in future material.
The others definitely seem expendable.
Varric had his heyday as Dragon Age 2's narrator, but in Inquisition he's definitely more of a standalone, companion type. And he doesn't seem like he'll be willing to tolerate everything.
Blackwall's theme is sacrifice, so I wouldn't be surprised if he even has some sort of quest tied with the chance of him dying.
Iron Bull is a mercenary. Mercenaries are often the ones that can get killed off easily. He also seems optional and could probably be skipped or refused in the party altogether.
When it comes to Sera...from what I've seen in the Qunari Mage gameplay video, it seems like her death is definitely a possibility. Judging by what's written about her character it also seems that she can leave quite easily, or even refuse to join your cause depending on your choices.
Vivienne...I think depending how you play the Grand Game and who you side with will ultimately decide where she stands. I'm thinking if you side with Morrigan, go against the Empress, or even establish an Anti-Circle position it's possible she leaves.
Don't know anything about Dorian, Cole, and Solas other than their existence, but from what little I've read about Dorian it seems like he's both optional and can be killed off (that scene with Alexius looks significant. Maybe its possible to betray Dorian for more power?). Can't provide insight about Cole/Solas unfortunately except that I can see Cole being sort of like Zevran, in that he attacks you and you have the option to kill him. The Last Supper speculation also suggests he plays a Judas role, so maybe multiple opportunities of dispatching him?