[Gloppagus]

Name: Weather Lover

General Description: A pokemon that can easily thrive in all 4 weather types via forme changing.

Justification:There are 2 things Pokemon as a franchise does well right now.
1. Weather.
2. New formes of Pokemon with different stats.
Everyone is getting sick of the same old weather wars, so why not shake things up quite a bit? The idea may seem overpowered at first, but with varying sets of stats, its function in any weather (or no weather) can be entirely different, even to the point that if your own weather goes down, your EV spread and moveset can end up screwing you over.

Questions To Be Answered: How can a mon thrive in all four weather types at once?
Is a changing BST even remotely usable in a competitive situation?
Will the mon be able to function in a multi-weather situation?
Can the mon be used effectively outside of weather?
What sort of BST would be weather appropriate?
How wide should a movepool for a weather warrior be?
How will the forme changing work?

Explanation: Imagine Castform (or Rotom to a lesser extent), but better. Depending on how you want the forme changing to work, it could be either a usable Castform, or a weather-oriented Rotom. Basically, this Pokemon can either be too versatile, and spread itself too thin, or be too niche, and be completely unusable after a short amount of time.
Just think: a Pokemon that has the ability to change it's secondary (or only) typing and base stats depending on the weather. It could easily be its greatest asset and weakness at the same time.