I think character meeting characters that Ash has befriended or became rivals to is a great idea that shouldn't be used every four years. If there were to be a new series for Pokemon, I suggest that the story could use more characters from the past 15 years and even those characters meeting others.
I have a list that I find character interactions interesting:
Kasumi meeting Hikari: Hikari has indeed took an interest to Satoshi's lure of her, but I can see some companionship in each other because of their differences. Otherwise, they can become friendly rivals for anything in particular and have their personal opinions towards Satoshi.
Kasumi and Airisu: These two have many things in common especially pesturing Satoshi. I can see both of them tag teaming to tick of Satoshi whenever he does anything childish.
Shinji and Shigeru: They are indeed rivals to Satoshi and by far the most intense, their charcters are different enough to show a possible rivalry within one another or just for personal interest.
Kasumi and Shinji: Their characters don't collide very happily. The neglectful, internally confused, and cold-heartedness of Shinji towards a mood-swinigish, and mature (with a mix of childish) textures of Kasumi, I can see Kasumi being a helpful person towards the development of his character which has many questions left unanswered.
If the writers would add something similar to these ideas, a future series can be very interesting and different.