I agree with Inversed below about Gerads contributing.
In terms of a transition - We've seen a couple of major transitions in the past 4 years - Snyder to King, with the Rebirth special, Jurgens and Tomasi to Bendis, with the Superman and Action Comics specials. But I think King's Batman run is a bit different, since he's transitioning to a more focused stage, rather than leaving entirely. And Bat/Cat, though I bet it will stand on its own, will also be able to be read as a new chapter in King's run. Which is very different from what Snyder did with All-Star Batman - it doesn't feel like "the next chapter" of his n52 run. We'll have to wait and see, but I'm betting we're just going to see Tynion on 86, and King on Bat/Cat 1, and go from there.
I would be very curious to see if all 12 scripts are done. That would be quite impressive, unless he is mostly adapting previous plans. But that seems unlikely, since he was planning with Mann just in May, and he tends to be very improvisational in his scripting sometimes.
Agreed - #85 is oversized (48 pages, though I always forget how many story pages that is - probably at most 38), so I'm betting we're going to have a sort of "greatest hits" type of thing, similar to Batman Eternal #52, where you have different artists contributing different sections that hopefully celebrate the impact those artists have had on the run.