How would you change/better balance Ice type Pokemon, aside from making them resistant to water-type attacks?
Well… I think Ice should be bad defensively; I think that works as a type identity thing. It doesn’t need to be as bad as it is, though. Resistance to just one other common, strong attack type probably makes it about as good defensively as Psychic, which is a poor defensive type but not actually comical, and supports a decent variety of tank and support Pokémon. Water… well, Water does seem like the most logical choice there; it’s not perfect, because one thing that Ice-type tanks need is a point of distinction from Water-type tanks (who both resist Ice attacks and can normally learn them). Maybe there’s an argument there for resistance to Dragon (although at that point you probably need to give Dragon a buff somewhere else) or Ground, instead of Water. I know you said aside from that, but I really don’t know that there needs to be much more, at least not in terms of adjustments to the type chart itself; Ice is also really strong offensively and I don’t want to risk overtuning it. I like the more indirect buffs like the addition of new Hail synergies – Aurora Veil, Slush Rush, Ice Face – and I like the suggestion in the comments of this post that Ice-types should get a physical defence buff during hail, to parallel the special defence buff that Rock-types get during sandstorms.