Rainbows bring good luck. Like the pot of gold at the end of the rainbow. So there should be something special in the game when rainbow appears. What would you like to happen in the game during rainbow weather?
- first Pokemon that you encounter and catch becomes lucky
- higher chances for getting lucky Pokemon when trading
- you can encounter a special Pokemon carrying a gift and when you catch it you get rewards
- you can get a special gift that gives you rewards from Poke Stops or Gyms
- higher chances of encountering shiny Pokemon
- you get double candy/stardust/xp for every Pokemon you catch
- something else (write in the comments)