This basically is the plot. Magekiller is a series of side stories with Marius and Tessa as the connecting thread.
Which is an OK approach for a continuing series, but I wonder if it was the best choice for a limited run.
I'm just biding my time to the next arcs, honestly. Perhaps those will be more entertaining when not constrained by the game events and the various doings of the Inquisitor.
So far it's just "Hey, look at these previously unmentioned characters and how they were totally involved in the Inquisition storyline without you knowing!" But that isn't enough, it isn't adding anything and we aren't learning anything interesting or new (Aside from one new word). That's why it feels more like we're still establishing character backgrounds.
Yepper, completely agree.
Did they mention what was so all fired important about the Hissing Wastes? If the Inquisitor had already been there, then that means we had already cleaned out all the old dwarven tombs, nothing of which I would have thought was of interest to Corypheus or the Venatori even though that is what they were investigating when the Inquisitor arrived. So it would seem that after we had destroyed the main Venatori base in the region and all their outposts, they then thought it worth their while to return on mass and take up residence again. What for?
Yeah, isn't it nice to know that our cleared out areas don't actually stay cleared out, and that we don't actually resolve anything? 
One issue left? So much for exploring what happens after the game ends
There will be other plot arcs, like with the first run of comics. Those will explore the post-game events. This first plot arc was always meant to cover just the pre- and peri-game events.
I'm going to assume that the final issue will mention Corypheus's defeat and show our duo off on another adventure, to be expanded upon SoonTM.