The goal here is to fix bad or underwhelming pokemon with as little tweaking as possible. They don’t necessarily need to become Uber tier gods but they should have a niche at least. Try to avoid changes to the Pokémon’s base stats unless a small alteration is all that’s needed, custom moves/move pool changes, new abilities, and signature items are on the table though.

I’ll start with castform
>Forcast change: castform now has a sandstorm form and the ability now gives wind moves +1 priority
>Move pool changes: Add tailwind, heat wave, icy wind, petal blizzard, and air cutter
>New item: Wind Sock, raises attack and special attack one stage when tailwind is used
>New Moves: North Wind, South Wind, West Wind, and Easy Wind; 90 BP, 90% accuracy, ice/fire/water/ground type respectively, wind moves, sets or extends its respective weather for 5 turns or 8 with the appropriate rock even if the move misses. East Wind has the added effect of hitting flying/levitating Pokémon
Now it has the use of being a weather setter and tailwind user that doesn’t rely on switching in to set weather and it can tack on +5-8 turns of weather onto a weather effect. This lets it work as a priority tailwind setting (murkrow and tornados do that better) and a secondary weather setter than that extend a weather effect to resemble Gen 5 weather.