That is actually not peace and happiness, as was noted in Ghost Story. Purgatory (or whatever you want to call the in-between) is for people who can't pass on, because they are not at peace with their fate.
I'd think, on her way through Purgatory, she picks up her dad, and they move on together. If, you know, it happens at all, which I really doubt.
I also doubt she'll be turned into a Valkyrie, or anything supernatural. There's not much to be gained narratively by doing so, except to screw up her relationship with Dresden. At this point, if that happens, why bother even writing the relationship at all? It'd be a staggering anticlimax to happen in the next book or two.
No, I expect Murphy's role to be significant, but not on the front lines. She doesn't need to be fighting at Harry's side; he's done the Battle Couple thing with her, Susan, and Luccio often enough that it doesn't bring anything new to the table. And, as I've said in way too many threads already, I fully expect her to be involved with Tilly forming an FBI version of SI in Chicago, as Tilly's liaison with the supernatural community.
Aside from that, don't discount her role with Maggie. Dresden's daughter is going to need someone a bit more stable around, like she had with Michael and Charity. Harry's first experience with a stable environment was with Ebenezer, and by then he was sixteen and already had two burned building under his belt. I fully expect Murphy to become the "favorite aunt" she's frequently described as. She is, after all, the second person shown holding Maggie, and the only one Dresden relinquishes her to after Chichen Itza. I'm not saying she settles down to be a stereotypical housewife or anything, because that isn't Murphy, but I'd like to see how that relationship develops, and if she has any involvement in the YA series, maybe as the person Maggie calls instead of Dresden because she knows Murphy won't blow a whole through the school if she needs help.
Kind of the reverse of the Molly-Harry relationship, where the firstborn daughter sees a normal person show up and do normal things instead of a wizard showing up covered in blood.
After The Warrior, where Jim kind of hammers home the idea that the Fight doesn't stop because the soldier isn't increasing his body count, I really, really doubt he'd have Murphy get powered up.