Since it's only a model... (Sssh!)
I think the big difference between government structures, whatever -ism they go by, should be the amount of time it takes them to cycle from efficiency to bureaucracy or from honesty to corruption. The more skilled your civilization becomes at governance (through researched technology), the longer it takes to move from a high point to a low point.
This would let you pursue more than one path of governance. Unethical governments might consist of varying forms of Tyranny, Fascism, Corporatism, Communism, or Fanatic Thearchy. Ethical governments might consist of varying forms of Democracy, Meritocracy, Benevolent Dictatorship, Populist Royalty, or any number of theoretical forms of government that turn to mud when they encounter the real world (Anarchy, Communism, etc). In between you could have similar choices, say Feudalism, Elitism, Hereditary Democracy, etc.
Set whatever maximum or minimum numbers you want to represent the efficiency (vs. bureaucracy) and honesty (vs. corruption) of each type of government. Researching social technologies would enable you to move up the ladder. You could shake up an inefficient or corrupt government by either paying a "restructuring fee" from your treasury or by going too far into debt. The cost for going bankrupt would be reduced morale and production for a number of turns determined by how far in debt you went and how far along the cycle of entropy your government had already travelled.
Doing it like this would give you a bunch of government technologies to choose from and set the stage for some interesting random events. For examples:
1: One of our social scientists has come up with a way to streamline existing bureaucratic channels without tearing the government apart and starting again. Stock markets have already risen on news that we will be implementing the new streamlining measures. Investors everywhere are cheering your name!
2: We have compromised the Diamond Tong Trade Cartel! Kickbacks from the Cartel will boost our economy and we can expect a slight decrease in social unrest now that these dogs have been called to heel.
End transmission.
~SDC~