I am going to approach this question differently by asking a different question. WHY would JB have the story end with Faith.
* Part of it is symmetry of course -- the really short story with Faith was the first Dresden story. So having her play a role in the last story is symmetry.
* Part of this is that Dresden has a relationship with Faith -- a connection. That connection has to be important.
* Part of this is just a reward for us fans -- who obsessively read everything that JB writes.
Symmetry is doubtlessly a part of it.
And fanservice may play into it, but I think it'd be more like giving Harry a win. The girl he saved years ago returns, and Harry's overcome with emotion, because of course he is.
His connection with Faith was one he had made when he was a very, very different man. Since then he'd seen and fought werewolves, ghosts, vampires, Faerie Queens, weird, shaggoth-like creatures, ghouls, demons, Fallen, and more dark wizards than you could shake a stick at. He fell in love, twice, and... assisted the first's suicide in a horrifying act of murder and genocide. He died, and was forced to come to terms with what the world would be like without him. He had a daughter of his own, whom he had to pull out of the vipers' nest, and has a less-than-traditional relationship with her. He's been tempted by a Fallen, and now has to battle the base impulses of the Winter Knight.
And Faith has had a whole life we haven't heard about, whether inside or out of the Supernatural community.
In all, if they had soulgazed, I'm willing to bet they've changed so much that they'd be able to do it again.
When Faith arrives, I think Harry will have a momentary emotional break. He'll remember the man he used to be, and look at the way he is now, and he won't like it. And, honestly, I'm looking forward to that kind of introspection.
I don't know what her role is going to be. If she brings him a case (which, to me, means he's back working as a PI and no longer the Winter Knight), I don't expect her to be a person of power, just someone desperate who remembers the man who helped her once.
If she turns up as a body, then I will personally write Mr. Butcher a very angry letter that I'll never send.