Episode 10 was just fine, that Alto Mare cameo confused me more than making me nostalgic bc I thought it was clear this Latias isn't the same one from the movie, is she her successor? Also why they don't trust humans? Aren't Bianca and Lorenzo taking care of them like they did with the previous Lati@s duo? Eh, not a fan of leaving things open ended here...
Yeah, this actually felt like a last minute decision cause there been plenty (and could have been plenty) of opportunities not just in these episodes but during Journeys (the writers knew they were retiring Ash) to feature a Legendary and have Ash think back to his movie adventure with that character:
Mewtwo (M01 & 16):
The only other confirmed movie cameo, though at that time was purely fanservice so I thought nothing of it. Cause otherwise, if they had planned ahead to do some quick cameo of movie characters, at the end of the episode they could have Ash also think of the other time he had an adventure with Mewtwo and we see a scene of the M16 Mewtwo visiting with the Genesect Army in the Absentia Natural Park.
Lugia (M02 & Whirl Islands):
Another one they could have had a two-fer with. After the Lugia Raid episode could have had a quick scene of Melody practicing her ocarina looking out onto the three islands & one with an slightly older Oliver riding on the back of an also slightly older (though much bigger) Silver.
After resolving the problem of a group of Unown which were keeping a town in permanent night due to a little girl not wanting to part with her Cleffa friend, could have seen how Molly was doing (since she'll be slightly older, maybe show she is now on her own Pokemon journey and show she caught the early stages of some of the Pokemon she used in the movie).
Lucario (M08 & Riley):
Ash got a Riolu which evolved into a Lucario and connected to it via aura. Now the main standout NPCs of that movie were Sir Aaron and Lucario, who are dead, so I guess we don't need a cameo of other characters from that movie. But what would have been nice to see was Riley. Would have been an easy setup too: though connected via aura Ash and Riolu are having problems syncing up. Ash remembers about Riley and goes to see him and ask if he could help and Riolu get on the same wavelength. And hey, Riley in the anime is an Aura Guardian (infact in the episode they met him he said he was in-training, so they could reveal Riley has since become a master). Maybe have Ash help Riley with some Aura Guardian business, and as a more synced Ash & Riolu leave back to Cerise Labs, we could fade to inside Cameran Palace where we see Queen Ilene and her attendants walking past the painting of Sir Aaron and his Lucario.
Dialga, Palkia, & Arceus (M10, 11, & 12):
I forgot that the Sinnoh Journey episodes are actually their own specials called the "Arceus Chronicles". Well, anyway, in the last episode when the day has been saved and time & space has returned to normal could have seen some scenes/characters from those movies. I'm imagining for M10 just a quick shot of Alice & Tonia in the garden with Darkrai hanging about. And they could combine M11 and 12 together by having Newton (and maybe a casually dressed Zero, showing he has reformed) meeting with Sheena and Kevin showing them the Time-Space Axis.
And from there if they wanted to include cameos from other movies they would have needed to have an episode with the Legendary or Titled Pokemon in it.