spoilers:end of spoilers
It's another good thing to highlight how AFO treats everyone, even his most loyal subordinates, as mere pawns. That Gigantomachia does genuinely care for his master, so to see him just abandon him after not being able to see him for so many years, would greatly anger him. Not being able to comprehend that even he has feelings, all that matters is what's convenient for him (letting him get captured and breaking him out later). What I'm expecting currently is we may end up seeing AFO steal Machia's Quirks, which will not only effectively de-power him for good and allow the Heroes to easily deal with him once the battle's all done, but also have the "final boss" be a giant Titan-sized AFO.
The movie character cameos are very cute to see, which I think is the best way to do it. Making them full characters might be a bit too much baggage for people who have never seen the movies (Shirakumo already kind of is a problem regarding that), but having them show up among a crowd of other extras is the smart approach, doesn't require any extra knowledge, and if you recognize them, it's an extra treat for you.