I would have Hugh battle the PC in Join Avenue, optionally in LostLorn Forest, and the top of the Celestial Tower. Also, instead of having a Monkey, Bouffalant and Unfezant, I would have him have Amoonguss/Camerupt/ Walrein to fill in the two remaining elemental types, and having a Grumpig.
I would have Silver have a Houndoom instead of a Kadabra.
I would have Wally have optional battles with you after you get the Dynamo Badge to show his progress. I would have him battle you at the end of Meteor Falls, and once more in Lilycove, while having your second to last battle with him in the Cave of Origin, due to his caring attitude towards pokemon.
I would have Lyra/Ethan battled you when you received your first pokemon, and again after leaving to the Pokemon League in New Bark Town. I would have him/her have the remaining starter pokemon, Marril, Noctowl, Ampharos, and Jumpluff.
Description: A huge amount of rainfall is summoned. It makes the weather rainy. It never misses.
Description: The opponent is sprayed by an immense amount of toxic acids. It makes Steel types weak against Poison type attacks.