With a hypothetical situation like this, I would use the imperfect with "if" and then the conditional.

"What would you do if you were rich?" is the most common way to say it.

"What would you do if you became rich?" "What would you do if you were rich?"

'What will you do if you become rich? ok 1st conditional > Suggesting that there is a possibility that the person can be rich one day. What would you do if you were to become rich? 2nd conditional >This is just an imaginative scenario of him being rich one day.