Question, are you sure it was the Archon of the Imperium who asked for Inquisition help with the Venatori, or was that the King of Nevarra?
I wouldn't say she has influence over the Imperium, just that she would have some influence among the lower classes within the Imperium's society. Also, the Archon and Magisterium would likely want to deal with her one way or another. Like you wrote, she could be an outlaw that the Imperium recognizes as a threat. So I see them as either trying to appease her, or taking the risk of diverting resources from holding back the Qunari in order to attack Calpernia and her Venatori forces. Or maybe they do both. They attack her first, then when she proves formidable they try to make peace with her rather than waste more resources that would be better spent fighting the Qunari.
Did you read her short story? Granted, I don't know if it's considered canon or not, but I think it was written by the same writer who created her. She is shown to be well loved among the lower classes, especially slaves. The game also shows that she commands the Venatori. So they will definitely follow her and her vision of a restored Tevinter. With them as well as numerous slaves joining her cause, she could grow to be a major threat to the Magisterium. She also already pwned a magister in the streets. So her magical skill is not to be messed with either.
If what you wrote about the Archon is correct, he considered the Venatori a threat. Thus, not a fringe group. Also, it was led by the main ancient magister darkspawn. That's hardly a fringe group.
What she thinks matters to those who follow her, and those who would yet follow her. She would recruit more followers by sharing her ideas and visions with them. The Magisterium is fragmented though. They have agendas against each other. They have their own cliques. Calpernia isn't just one woman. She has various Venatori with her. She has former slaves. These slaves may have secrets. As a former slave herself she knows how to work the magisters and their games. She could send assassins and spies disguised as slaves to magisters that she needs to eliminate. There's all kinds of fun things she could do. So are they a complete nation? I don't think so. And even if they could unite against her, she's young and they're mostly old. She could outsmart them. Be unpredictable. Think up new ways to dominate that they would not have considered or planned for. They would be stuck in their old way of thinking, trying to protect a crumbling nation and their own excess. Calpernia and her followers would be fighting for a new future of restored glory, prestige, and freewill. What happens if children start turning on their parents, like what happens with Dorian and Halward? Calpernia could represent a dangerous element to the Magisterium.
Regardless of how we see Dorian, he can possibly return to Tevinter with no consequences. But the way I see it, he helped destroy an ancient magister who brought the blight into the world and tarnished the Imperium for ages to come. His accomplishment should play well in Tevinter, even if people continue to see him as a pariah.
Anders has been known to exaggerate. But if the Magisterium rushes to kill Dorian, I think Calpernia would soon find herself another useful ally in the war against the Magisterium.
The settings are indeed bad, but that's what makes for an interesting storyline.
If they do this plot, my theory is that they will equalize Calpernia and the Venatori in both the mage and templar paths. Meaning, a Calpernia of the templar path who led the Venatori will find herself with a crippled and demoralized force that she will have to build back up on her own. Conversely, in the mage path, she will have always been a lower ranking commander of a small group of Venatori that didn't see much action, and feeling that her true skills were never properly utilized by Cory (who chose Samson), she will set out to make the remaining Venatori into a force to be reckoned with.