I don’t see how it breaks anything if it’s opt-in. By default you get the current behavior with zero value initialization if that’s what you want (and in many cases it is). But if you’d rather force an explicit value to be supplied, what’s the harm?
If it would only complain on struct literals that are missing the value (and force a nil check before access if the zero value is nil to prevent panics), that would be enough for me. In that case, your Zero function and reflect.Zero can keep working as-is.