@mkopriva是对的
但是，如果愿意，可以使用反射动态构造新的结构
下面是一个示例：

package main

import (
    "encoding/json"
    "fmt"
    "reflect"
)

type DynamicMap map[string]interface{}

type Product struct {
    Name        string            `json:"name"`
    Code        string            `json:"code"`
    Description string            `json:"description"`
    Brand       string            `json:"brand"`
    Variants    map[string]string `json:"variants"`
    Categories  []int             `json:"categories"`
}

func (j Product) MarshalJSON() ([]byte, error) {
    m := DynamicMap{}
    t := reflect.TypeOf(j)
    v := reflect.ValueOf(j)
    for i := 0; i < t.NumField(); i++ {
        if t.Field(i).Tag.Get("json") == "variants" {
            dyn := v.Field(i).Interface().(map[string]string)
            for key, val := range dyn {
                m[key] = val
            }
        } else if t.Field(i).Type.Kind() == reflect.String {
            m[t.Field(i).Tag.Get("json")] = v.Field(i).String()
        } else {
            m[t.Field(i).Tag.Get("json")] = v.Field(i)
        }

    }
    return json.Marshal(m)
}

func (j Product) UnmarshalJSON(data []byte) error {
    fmt.Println("Unmarshal...")
    return json.Unmarshal(data, &j)
}

func main() {
    p := Product{Name: "aa", Variants: map[string]string{"a": "a", "b": "b"}}
    marshal, err := json.Marshal(p)
    if err != nil {
        fmt.Printf("%v\n", err)
    }
    fmt.Printf("%s\n", marshal)
}

{"a":"a","b":"b","brand":"","categories":{},"code":"","description":"","name":"aa"}