Freddo Obama has no actual 'power' per se in govt as a former President. But he could have enormous influence with the public and foreign leaders.
And removal from office by means of impeachment is only used on a sitting President for treason, bribery, or 'high crimes and misdemeanors'. The grounds for Bill Clinton's failed impeachment was the crime of 'lying to Congress'. Unfortunately unpopular policy will not be enough for Trump to be impeached. There must be some criminal or unconstitutional behaviour or action. And unfortunately Trump has a 'yuge' team of advisors and lawyers who will be sure that he stays on 'just this side' of the Constitution. Removal by unfitness for duty? He may be an unprincipled, greedy prick, but he's not insane.
No, unfortunately I think we're stuck with Madman Trump until 2020. Our only hope now is the 2018 midterms. We need a Democratic majority in the Senate and/or the House of Representatives. I can only hope that in 2018 there is a Democratic equivalent to the 'Republican Revolution' of 1994.