Most people do not consume enough potassium. The RDA for potassium is high, and potassium deficiency seems to cause bad things like strokes. You'd need to eat ~8 bananas a day to satisfy your RDA (which isn't that surprising - the dastardly banana lobby has tried to cast bananas as high-potassium, but e.g. tomatoes have more). And excess potassium probably isn't very dangerous. Also, someone on LW (Kevin?) reported a nootropic effect from supplementing potassium.

Most people consume too much sodium. (There's been some uncertainty around whether excess sodium is actually bad, but it still seems clear that we consume more sodium than we need.)

Potassium and sodium can both be eaten in salts, which will taste pretty similar. Therefore, perhaps we could make health gains by replacing much of our table salt with potassium! Indeed, some people have to do this for health reasons, so the great machinery of capitalism has already done lots of work for us here. For instance, here you can buy 12x3oz of potassium salt (enough to last more than a year) in shakers for $15. I've been trying this out for a while and it tastes almost like normal salt.

I don't know much detail about nutrition, though, so this may be stupid for reasons I haven't thought of. Could someone who knows more about the relevant science please weigh in?