Fixing Serena:
>Have her brag about her pedigree more often and talk about her Ace Trainer parents
>Have her get pissy when she loses, saying things like "I-I wasn't serious! I wouldn't really go all-out against a random who just stepped off the plane"
>Have her be a giant shoulda-woulda-coulda about things like the Mega Bracelet (which you defeated her to win), defeating Team Flare and becoming Champion
>Eventually, she should confess that the reason she races ahead of the group (or stays behind in some places) is because she had become obsessed with defeating you and was too proud to ask for your help
She sort of did do the above, but I would've turned it up a notch desu
Mind you, I would've also cut tourist photos in favor of an upskirt snapshot sidequest, but whatever