The system depends on the structure of the country. The bigger countries who have a federal structure tend to have presidential republic. The smaller countries are better off with a parliamentary republic. But of course, any country can change its system if one party has enough majority to change the Constitution (like Nepal now). But in Zimbabwe's case this is impossible. Yes, Mugabe is quickly losing his grip on power, which could make him very dangerous because noone can predict how he will react. There could be either a crackdown on the opposition (which is already happening partially) or his own party will pressure him to step down, which is the better option.