It's possible, but while we've been told the companions are less extreme in their viewpoints than DA2, they still need to be relevant somehow. When it comes to Solas in particular, we know too little about him to really begin to determine what he may or may not be involved in. And Masked Empire implies that what's going on with the elves isn't going to be insignificant, and if so, it's likely one of the elves in our party is going to touch on that. Solas is simply most likely all considered. Maybe not necessarily as an "advocate" for anything, but he may end up significant in that storyline. Or maybe he's trying to help the mages. Or both.
Or maybe he's just a hermit-y elf mage fascinated by your glowy fade hand and it doesn't go further than that.
It's easier to come up with theories than to disprove things right now. 
And I don't know that Vivienne *needs* a foil, but we know there's some other mage in the party she doesn't get along with. It might just be severe fashion disagreements, but given her personal politics, I suspect it isn't that simple... Might not even be Solas, but...
Theories are all we have.

It just seems to me that apart from everyone expecting companions to represent factions, there is really no reason they need to support any cause in particular from the outset. They just need a reason for being in the Inquisition. Solas and Sera can play a part in the elven rebellion story without a reason other than they happen to be elves and if their character growth is linked to that storyline it will feel more organic. Solas is relevant to the story by virtue of being a Fade expert. Sera has access to a network of thieves. My point was do they really need to come to us wanting to help X group? I don't think there is any reason they need to, other than that story is easier to write. Which means Sera can not care, and Solas can be oblivious and the rebellion can still play a large part.
As for Vivienne, regardless of her political views I'd be surprised if there weren't companions who didn't like her based on what we've seen of her personality. To me she comes across as too self-interested to be presented as a fair representative of the Circle, which I take to mean she doesn't need an anti-circle apostate as a counter. She is already telling players that the system isn't perfect just by existing.
EDIT: Hold on. I think you thought I meant that they wouldn't be connected at all, and I thought you meant that they could only exist within those plots, so we may have made some overlapping arguments. The important thing is that I know what I'm talking about even if nobody else does. It's been a long day. Sorry.