A weakness of zombies could be water and I think they should react just like how cats do to water, so they don’t really mind rain but they don’t like rivers and pools etc.
What would fit with that would be water guns:
you can find water guns at swimming pools and beaches.
you also need to pump them up so their range is further.
and I think every zombie should have a different level of how much they don’t like water.
and if you become a zombie the only thing that you have is that you can’t swim.
I would actually say, that the zombies should try to find shelter, when it’s about to rain.
If they are afraid of water guns and not from the rain that wouldn’t make so much sense, would it?
And also, since you’re talking about water guns. I like the idea that you have to pump them up to shoot further. As for the zombies that don’t like water, my opinion is that they have to get slowed down and make some noises or something similar to that.
This would be super impractical to implement, and wouldn’t add anything to the game. If this idea were to be implemented there might be a couple of different levels of vulnerability to water, but there’s absolutely no reason to have so many possible reactions that each zombie in the game will behave uniquely.