A couple more speculative guesses.. Maybe they'll record What Lies Ahead as a full band arrangement. Peter seems to want to still tinker with it. Maybe they could film opening campfire material, then transport to the live gig.
Feels like cutting to studio footage midway thought the set would take you out of the live experience, but then I'm sure Peter could find a way to make it work. You wouldn't need the whole band together though for just and and/or bonus interview footage etc...
Personally I like the idea that maybe they're going to try out a few new songs that could potentially make their way into a revamped setlist for a second i/o leg, and they're going to film it for potential Blu Ray extras if some songs don't make it to the stage. Historically Peter likes to tour more than he does release new albums, but obvs this is all pure speculation. They may be filming another "live in the Big Room" session for an extra.
Ultimately, any activity from any of the main Genesis 5 is a cause for fans to celebrate.