"From left field" is an idiomatic expression that refers to a surprising or unexpected event or idea. Its antonyms would be "predictable," "expected," and "routine." These words denote a sense of regularity, normalcy, and predictability. For instance, when a person follows a well-established routine or a plan that is organized and expected, it is considered predictable. Predictable ideas or thoughts are those that conform to expectations and are not surprising or unusual. Therefore, if one thing is not "from left field," it is fair to say that it is something that is traditional, regular, and ordinary.