what do 你 think will happen if misty and serena had a rivalry over ash?

I doubt anything like that would happen.
Serena is the only female main character who has a canon crush on Ash.
About the other girls, including Misty, there are only fans' speculations.

Even if Misty was interested in Ash, the girls wouldn't hate each other.
Since 神奇宝贝 is an 日本动漫 for kids, things like hate is something the director rather not use. They don't want to teach children to hate. Especially not for a stupid reason like a 爱情 triangle.
Besides, I've seen tons of 爱情 triangles where two girls are in 爱情 with the same guy. They're "rivals", but they're still 老友记 and care about each other.

But /if/ Misty and Serena both liked Ash, they would most likely be friendly rivals.