what do you 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 Pokémon is an anime 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 pag-ibig triangle.
Besides, I've seen tons of pag-ibig triangles where two girls are in pag-ibig with the same guy. They're "rivals", but they're still mga kaibigan and care about each other.

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