Think just literally, what - 무엇, 무슨, 어떤 일?, and if 만약. So, what if can be (만약) ~ (한다)면 (어떨까)....? What if I don't want to go... 내가 안가고 싶다면.? 내가 안간다면? / 안가면? What if I don't have money? 내가 돈이 없다면? 돈이 없다면? / 돈이 없으면? What if I want to do it. 내가 그걸 하고 싶다고 하면? 그걸 하고 싶다면? / 그걸 한다면?/하고 싶다면? / 한다면? Of course omissions possible.

만약 would be the best answer 만약 하고싶으면 어떡해? What if I wanna do it? 만약 가고 싶지 않으면 어떡해? What if I don't wanna go?

And here are my best guesses for the three examples you gave (although I'd wait for a native Korean speaker to respond ;) ): 저는 안 가고 싶다면 어떨까요? 저는 돈이 없다면 어떨까요? 저는 하고 싶다면 어떨까요?

That's a great question and I wanted to know, so I looked it up. I found this structure: -다면 어떨까요 = I (wonder) what if... Check out this page for more ideas: http://dic.daum.net/word/view.do?wordid=ekw000184962&q=what+if