Do you think making water pokemon weak to poison type attacks would both improve game balance and be thematically sound? I believe it makes sense since pollution tends to muck up a water’s flow and is a great danger to sea creatures (which most water type pokemon are). And for that matter, why isn’t steel weak to electric? It’s conductive just like water, and it would give electric types more uses and balance out steel’s massive defenses.

On Water/Poison, I agree that it would make at least as much thematic sense as any of the other type matchups in Pokémon, and if you had asked me this a year ago I would have said yes without a second’s thought because Poison-types needed more love (I think I may actually have suggested this change myself at one point).  Today, though… well, Poison has already gotten a boost from being strong against Fairy.  Likewise for Electric/Steel, it would have made sense, but Steel’s just been nerfed by losing its resistances to Dark and Ghost attacks, while Electric-types have just been given immunity to all forms of paralysis.  I’m not prepared to say that these are bad ideas by any stretch of the imagination, but I want to let the new changes settle and rethink my priorities here before I start commenting on more adjustments.