I have been playing OU a lot the last couple weeks and I noticed that rain teams are very powerfull (and for that reason very common in OU). Most of the times (read as always) people bring Pelipper - Kingdra - Ludicolo - Wobbufett - Dugtrio - Scizor. I do believe that rain teams these days, especially paired with trappers, are too good in the current meta and therefore requires a nerf.
This could be done by banning the trappers or by adding a weather + speed ability clause. This means that teams are not permitted to include Pokémon with the abilities Drought and Drizzle with Pokémon with the abilities Chlorophyll and Swift Swim. They may enter a battle with one or the other.
This also shuts down sun teams. I don't know how problemetic sun teams so maybe its possible to just have this rule for Rain teams?
Looking forward to feedback from other people about this.