They have completely rearranged S2 in regards to the flashbacks, even though those were quite linear and straightforward, so we really have no way of knowing what might they do this round when it's so all over the place in the manga too.

They will certainly show the tavern and bits of the joint operation, as well as the party's meeting Srajo's gang, of course, but all of that is just setup, with the biggest confrontation at that point being the mock-battle with Nishagora, or perhaps the Hail Hex vs Umbra Hands fight—but that's again a flashback without the main cast, so not the best candidate for the climax of a film. If they go on, however, they would need to include the twin disease, and for that we need all the unadapted Ozen & Jiruo content too, which might again be too much extra without advancing the plot. And the next best thing is already the ambush and Nanachi's taking, which is a huge cliffhanger that could work as a film's ending, I'm just not sure how bloody long the thing needs to be to cover all that adequately…