"Return of Gary" is more realistic.
Plus Gary can travel with Ash to research Ash-Greninja and the secret of Ash-Evolution.
Ash & Serena are just 10 , They are too young for confessing & Dating !
One of the main flaw of Amourshipping is that its only important for Serena , Not for Ash or the series.
Even if Ash dump Serena for a better girl , It will not have any effect on him or the Anime.
Romance in Kids anime has to be something that doesn't need dating or confessing and influence the entire show!
For example : Ash & his Love-interest will be completely unaware of their own developing feeling toward each other until they become old enough to understand it.
Jimmy & Marina could be a good example of that .
You should also add "Return of May" considering ORAS!