That'd honestly be really cool. I don't know what I'd expect to see with this though. On the subject of weather, Weather Ball needs a buff I think, to at least 65.
Edit: Actually, I think Castform's ability needs to be changed to work with Misty Terrain, Electric Terrain, Grassy Terrain, and Sandstorm and get new forms.
weather ball gets 2x damage buff in any weather condition. this is why they give you a castform right before fortree, so you can weather ball the altaria before it dragon dances your entire team into shreds.
castform is actually really good for nuzlocke, he has a sturdy body and enough versatility to provide powerful super effective attacks.
he has access to fire, ice and water moves that all have 150 base after stab. not bad in the least bit. I have used this little gem before and I feel if he got a mega evo, he'd be played on the same level as slurpuff. A one trick pony, albeit a powerful one.