I don't think it'd be that strange for a new castform form. Think about DP. Magmortar. It's a new pokemon in gen4 but wasn't available in the main game (pre nat-dex), in fact there was a massive LACK of fire types...
But back on the point, there's no telling what they'll do. They're implementing new things every generation so it's not out of the question.
There's a difference between hail/rain vs sun vs sandstorm
all are weather but hail/rain are both precipitation, sunny weather in thr result of the earth's exposure to the sun, and sandstorms are caused by wind blowing around dust... (and google sandstorm and it's funny cos the first web page result it bulbapedia. =P)
anyway my point is that hail/raindance are both different from sunny day so it's not that strange for another different weather effect to get a castform form.
a fog move or form would be interesting. but fog hasn't really been implemented too much in the games. there wasnt many areas in sinnoh to use defog and there haven't been implementations in hgss of it either... i doubt it'll be used but ya never know. =D
sorry if what i said was kinda confusing. XD