回答：可以的。该口语表达的完整形式是：It’s a good idea. / That’s a good idea.
省略 it’s / that’s 即成为：A good idea.
进一步省略不定冠词，则成为：Good idea.
在实际运用中，几种形式都可以用，但以 Good idea 用得最普遍：
—The weather’s too nice to stay indoors. Let’s go out and sit in the garden.
—That’s a good idea.
—Why dont we go for a swim?
—A good idea!
—I’m tired. I’m taking next week off.
—Good idea, honey. You do need a break.