Well, I think there is going to be a lot of progression. For instance, I think starting out, we will become AN Inquisitor... not THE Inquisitor.
Likely, we'll see the Veil Tear. And all kinds to crazy baddies start pouring out. We'll fight with a group of others, seeing first hand the power of these never-before-seen demons and dragons. Our team will either die for being inexperienced (the Jenkins treatment) or wind up sacrificing themselves to allow us to escape (the Gorion treatment).
Then, the Seekes and Cassandra will arrive on the scene. Seeing that we witnessed these new threats firsthand, they come to the PC's aid and escape the city. Being the only one to fight these threats firsthand, Cassandra takes you to meet the head of the Seekers (likely Leliana). Leliana/Other Big Boss realizes this is a new threat which will require the unification of all of Thedas to beat, and says this can't be something that the Seekers as part of the Chantry can handle.
She then declares the Inquisition formed and you, as the resident expert on these new enemies, are joined into their ranks, with Cassandra to accompany you. As events progress, you raise the prestige and reputation of the Inquisition until, tragically, Leliana/Other Big Boss dies, then appointing you as the new leader of the Inquisition.
Boom goes the dynamite.