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The interviewer notes that Oak didn't appear at all outside of episode 3, and that Oak's lecture segment getting replaced with PokeTV when XY&Z started meant that Oak didn't appear at all throughout all of XY&Z (the interview was conducted before XY&Z47 had aired) and asks if there was a reason for this. Yajima confirms this was a complete oversight, not something intentional. Presumably this goes for other callbacks to older series as well.
When XY&Z started, Yajima knew that the series had to wrap up everything in only a year, and they did a meeting to basically set up how the events of that year would go. He also specifically says that this is the reason there's minimal filler in XY&Z, there was absolutely no room.
Yajima wanted to drop the use of stock footage completely, but this wasn't possible to do, so they went with creating stock foreground footage that could have different backgrounds added to it instead.
One of the things that makes XY and XY&Z feel so new and refreshing is how Ash is depicted as a very gallant and dependable boy, one that Serena admired and had a crush on even before they started traveling together. It's precisely because of these people that Satoshi and Serena's farewell scene in "No End to the Zeros! Till the Day We Meet Again!!"(The last episode) has become so famous.
