Please add a bet:
Will Trump in his first term be impeached by the House and then convicted by 2/3 of the Senate or resign?
With the Democrats likely taken over the House and their strong opposition to Trump, an impeachment in the first term becomes a serious possibility. The bet for impeachment is now between 40 and 50%. Conviction or resignation after an impeachment would have the same legal effect, the removal of Trump from office after serious accusations of High Crimes and Misdemeanors by the House. Both outcomes, resignation and conviction, have small probabilities, but their addition could bring the bet closer to the interesting 50% mark.
I withdraw my suggestion, after another poster pointed out that the bet if Trump will be still president on Dec 31 of 2019 is a good proxy of the chance that he will be investigated and removed from office, either by impeachment and conviction or by resignation like Nixon.