You can only be taxed on what you have in that moment. What you make after that day is not under the rules of the IRS. You can not gain citizenship in another country if wanted unless they accept you knowing this. Most countries have extradition treaties with the US. It is almost impossible now to make it happen unless you do something like the American who is in Russia in this moment and he never renounced. Renouncing means you will be checked. You are opening your life and also closing it at the same time. If you have something to hide it would be a bad decision to do this. I know a dozen gringos in Colombia who have renounced. They do very well and the process was quite easy for them.