All I want, as strange as it might sound, is for Ritchie to come back. I know. Ritchie. Such a minor, nothing character that people only remember as "that clone of Ash who only beat Ash because Charizard took a nap". But putting aside that I have a MASSIVE softspot for Ritchie, hear me out:
When Ritchie was last in the main series, he and Ash got tied up with Team Rocket and Lugia. Ash and Goh met at the start of Journeys (Pikachu episode aside) because of Lugia. Ritchie could show up in Vermillion City, chasing Lugia, and Ash and Goh could join him in chasing the giant silver ocean bird down to see Lugia again.
The last time we saw Ritchie in the side story episodes, he was on a solo journey that let him have a time travel adventure courtesy of Celebi and an adventure that let him see Moltres. I say just have him join either Project Mew or the PWC so that he can have an extra purpose to keep doing that, and so he can meet up with Ash and Goh sometimes.
And if nothing else, I think bringing Ritchie back would be a good way to show how far Ash's come. Ash isn't the same kid who got duped by Team Rocket and lost due to a disobedient Charizard anymore. Now, he's Champion of Alola with countless adventures under his belt, knocking on the door of a World Championship title. Have Ritchie return and battle Ash to prove that. Even if Ash lost to Ritchie again, it would prove that Ash lost because Ritchie simply a better Trainer, not because Ash is a bad Trainer.
That's all I really want out of Journeys. Nothing else. This is the one series where you can bring back an older, more obscure character without provocation and without it sticking out as awkward. Let my underrated forgotten boyo return.