I'm not really sure there is an equivalent for hypothetical questions in Japanese, but here's hoping there is.
In English, hypothetical questions are generally prefaced by "What would you do if..." or something along those lines. It seems like 「何しよう...」 would almost work, but that sounds more like "what could you do if...". To be really specific, how would one say "What would you do if you were dying?" or (for the sake of variation) "what would you do if you went to the park?"
I'm assuming 「公園に行ったならば、何しようか？」 is not correct because it sounds very unnatural.