What would you do if I thought you were being serious?
You shouldn't joke around like that.
What would you do if I said yes?
この場合、次のような言い方ができますよ。
ーWhat would you do if I thought you were being serious?
「私が本気にしたらどうするの?」
ーYou shouldn't joke around like that.
「そういうこと冗談で言わない方がいいよ」
ーWhat would you do if I said yes?
「私がいいよって言ったらどうするつもりなの?」
ご参考まで!