I too thought "think" was a variation of "thing," not the other way around. I stand corrected.

I agree that "think" makes sense as the completion of "If that's what you think...." But some contexts don't involve "think." For instance: "If you expect the CEO to be happy with these results, you have another thing coming."

"Thing" doesn't make a lot of sense in this construction, but it makes more sense than "think." So I'd say both versions are correct depending on the circumstances.