费致德《现代英语惯用法词典》对“What if...?”有以下解释：

What if...?的意思是：

a.“如果......那就会......吗？”（What will (would) happen if ...? What is (would be) the case if ...?）有时用虚拟语气，如：

Let people speak up, the sky won't fall and neither will you. What if you don't let people speak? Well then, you are pretty sure to tall one day. 让人讲话，天不会塌下来，自己也不会垮台。不让人讲话呢？那就难免有一天会垮台。

What if you go instead of me?（What would happen if you went to my place?）

b.如果......那又有什么关系呢？（What does it matter if...?），如：

What if it is (be) true?（What would happen, if it be true?）如果那是真的，又怎么样？

What if he does not come?他不来又有什么关系？