How about Ash have Tsundere rival like this-
Her Personality-
-She will act like an arrogant Princess !
-Her main Pokemon happen to be a Sylveon and she also has a Brxian .
-She's really Popular with Boys ! Even Clemont was astonish by her look .
-She usually act graceful and clam However But she get livid & angry everytime she see Ash or hear his name.
Example-
- She totally obsessed with perfection ! She claimed she could beat Ash within 3 minute in their first battle and even overwhelm him But Ash mange to break though her Perfect combination and lasted more then 3 minute before Team Rocket interfere .
-She is lonely girl ! After she got betray by her Best friend , She lose all her faith in friendship . It also one of the major reason behind her hate toward Ash Because seeing him always surround by Friend make her realize how lonely and miserable she is .
-She tries to intimidate Ash by various truant But Ash always ether misunderstand them Or flat out ignore them which makes her even more angry !
-Surprisingly , Ash is the first person to believe she has a soft & Kinder side! This believe become even stronger when he met her mother & learn about Past ! Ash often try to to drag her into various adventure and open her up to others
-Her mother is a popular model and Best friend of Diantha ! Her mother is more cheerful and friendlier then her Daughter ! She mistook Ash as her Daughter's Boyfriend and sometime she like to tease them .
-She defected Clemont before he left the Gym ! Clemont describe her as the Strongest Challenger ever to Beat him. However She doesn't hold Clemont in high regard and consider him as a trash of a Gym-leader .
-Although She start to develop feeling for Ash , She doesn't accept it until their big showdown in the league .
I think this how a Romance in Pokemon should handle !
Rather then making it a ''Subplot'' , Writer should try a romance that would be a part of the main Plot .