Yeah, but Castform dies to anything that just happens to be faster than it, has priority, has a good Special Defense, or can survive one turn and has a damaging move.
Also, I really want to make these weathers not just for one type each. Sandstorm, for example, benefits Rock, Ground, and Steel and is a playstyle rather than a type benefit. Rain benefits Electric and Flying types, Sun benefits Grass types, and Hail... doesn't really even benefit Ice types. Even Pokemon not immune to Sandstorm damage can use it well.
EDIT: Also, we should totally make a Fog inducing ability and move and make that a viable playstyle. Decreases accuracy, and something else?