I think Butcher will stick with first person past tense point of view. Can you imagine if the story were to be told in first person, present tense? That would be too annoying.
One of the worst trends over the past few years is the rise of first person present tense, the second most hated perspective (after second person future tense, which, as far as I know, has never been used in anything but flash fiction and CYOA stories, and might as well be mythical). It's super tedious and obnoxious to read, and doesn't add a thing to any text it's used in. It's my biggest complaint about the Iron Druid Chronicles, which I otherwise kind of just liked.
Anyway, I think the Big Apocalyptic Trilogy will be in first person past tense, but from multiple POVs. Jim has had a lot of practice throughout the short stories writing from Thomas, Molly, Murphy, and Marcone's perspectives (and Luccio? I haven't read any that will be in Brief Cases yet, so I don't know if hers was from her POV). I think the Trilogy will shift points of view from chapter to chapter—which will be fun, because none of the other shorts from other perspectives also have Harry as a character doing things (except for Thomas's, for like, a half a paragraph), so it'll be cool to see Harry do stuff from the outside. My favorite parts of Aftermath were Murphy narrating about the way Harry works and how weird it is.
Because the BAT is supposed to be the grand sendoff, I wouldn't be surprised if Jim let us see Harry be awesome from a spectator's position—make us sit back and watch the show for a minute—instead of reading Harry's constant narration explaining what he's doing, why he's doing it, and how it's happening.
It can still be reconciled with Harry's journals; have this be his project to gather
everyone's journals and publish a record of what happened. [WAG: That way, people will know and believe in Harry and his God Mantle will retain more power (like Mab's Brothers Grimm gambit).]