What If Serena Actually Went to the Alola Region after Ash left Alola to take part at Cerise Labradorites with Goh, I think she would decide to actually take part in Showcases and catch Alola Pokémon, and would now stay in Alola when Ash is still travelling to all 8 different PokeRegions.
Here's what I think what her Alolan Team would be
The Reason why I didn't add any Pokémon from Hoenn was because I felt that there wasn't any Hoenn Pokémon that she would have that relates to performing and some of them are already owned by other Pokémon Performers such as Kirlia, Mawile, etc. So I added Brionne, Steenee and Alolan Ninetales since they fit her so well, and I know, people will say that these Pokémon are already owned by the Alola Companion Girls, but here's the thing, I felt that Serena could have these Mons too and maybe she got reminded of them and later caught a Popplio and Alolan Vulpix, and got a Bounsweet as an egg then maybe Serena could have the perfect team and would be considered one of the greatest Pokémon performers of all time, and I think that would be quite an accomplishment for a companion of Ash's as a whole.