Well I want to know what is your opinion about Misty's and Serena's relationship??
This is kinda a weird question. I mean, in the canon, Misty and Serena have never met. Therefore their relationship is nonexistent.
I have voiced a lot of thoughts on what their potential relationship could amount to considering their similar romantic feelings for the same boy (though the version of Ash they each like is pretty different), Misty’s position as gym leader, talent as underwater ballet actress, Serena’s cute fashion sense/taste and her adoration for strong girls.
I think that them meeting and finding out the other has a crush on Ash would be awkward, would even admittedly create some natural animosity for a short while. The problem with being 100% certain is that there has never before been a situation similar to this one because both characters in this case have a longterm relationship with Ash; neither is a character of the day who will vanish forever at the end of the episode. Both Serena and Misty have been known to exhibit hostility/jealousy towards a third party making waves/moves on Ash after all. Neither is shown to be more capable around romantic rivals...
Once some time passes and the two are forced to endure each other’s company and they’ve butted heads a bit about whose feelings are stronger/whose relationship with Ash is better, something would change. They’d start getting to know one another as individuals. Misty would subconsciously begin to admire Serena’s baking skills, her sewing ability, her cute style. She’d also befriend Serena’s Pokemon no problem. She’d be intrigued by the amount of effort Serena puts into her performances.
Serena would probably be immediately/temporarily enamored after witnessing Misty battle. She’d hear some stories about Misty’s time as a gym leader, maybe even about her adventures during OS (whether it was helping Ash, saving Pokemon, playing Misty Mermaid, or winning the rare battle/tournament).
They’d transition into this awkward, instinctive place of respecting each other despite their competing feelings for Ash (and the fact that neither is actually willing to make a strong move regarding those feelings). Meanwhile Ash would be in the background, at first completely ignorant to their feuding, then eventually somewhat aware and confused, then just relieved that things seem to have overall blown over.
If they hung around each other long enough, I absolutely believe that everything would come to a head. They’d end up having an intimate conversation about their first impressions of one another, their acknowledgements that the other is much more/better than they were expecting, and would agree to be friendly despite their feelings for Ash making things a bit complicated.
If this were all a fanfic I was writing, I’d probably write it to the point where one acknowledges the other is best for Ash and perhaps find themselves at peace with the endgame despite some heartache, happy that they’d grown so much as a person and built such a strong relationship with another girl out of the situation.