I think the problem with hail is that it gives benefits ONLY to ice types, whereas the other weathers splash around benefits to electric, grass, ground and steel types. Any ideas to give other types advantages in hail?
It doesn’t even benefit Ice-types, really, so much as not disadvantage them… that is, unless they have Blizzard, Ice Body or Snow Cloak. I sort of think it would make sense to add something analogous to Sandstorm’s bonus for Rock-types and say that Hail gives Ice-types +50% physical defence. It’s hard to think of anything that makes thematic sense with Hail as a benefit for types other than Ice, though, which is probably why Game Freak has never done it. You might be able to come up with something that makes sense for Water-types, I suppose, but they already have bonuses from rain, so screw them. You could, however, make the penalties more severe for some types in particular – defence penalties for Rock and Steel, for instance, on the grounds that rapid cooling makes materials more brittle, which would add more to Hail’s ability to wear opponents down. Or perhaps Hail could do more damage to Pokémon that are weak to Ice attacks, which gives a bit more variance to how teams with different compositions are affected by it. At the moment the only real way to build a Hail team is to put several Ice-types on it, but if there are some Pokémon who are weakened significantly more than most by Hail, there will be others who are in a position to take advantage of that.