What is the Chinese equivalent of English idiom "As if!"?
The closest answer I can think of is "才怪".

A: "In a few years I'll be driving a Ferrari."
B: "As if you'll ever have a Ferrari!"

A: 过几年我就能够驾法拉利了。
B: 你能驾法拉利才怪!


怪 means incredible in English 惊异，感到奇怪

才 means having the ability (能力：～能。口～。这人很有～干)

This phrase 才怪 is evolved from the longer phrase "才奇怪" and the usage of which only becomes popular in recent times. If you are interested, you can read up this article which describes at length the historical development of the phrase.

Quoting from the article,

才怪逐渐演变出了一种新的意义：表否定。“否定”的意义是与“奇怪”义有紧密联系的，觉得某事奇怪，主观上本身就有不赞成之意。只有主观上认为与自己看法不相同才会觉得奇怪，出乎意料。所以，我们认为“才怪”现代常用表否定的意义是从“才奇怪”演化而来。

Translated to English:

才怪 slowly evolves to mean an expression of disagreement. The words "disagreement" and "incredible" are closely related. To think that something is incredible implies that the person does not agree that it will happen. Only when a person disagree with something would he find it strange or out of the norm. Therefore, we believe that the contemporary meaning of 才怪 is an expression of disagreement and evolves from the longer phrase 才奇怪.

In my own words, 才怪 has the same exact meaning as "as if it would happen".

Usage example:

A: I'm not going to smoke again, I promise.
我绝不会再抽烟了，我向你保证。
B: As if this is going to be your last cigarette.
这会是你的最后一根烟才怪！