One of fun things about Alpha Centuri was that you had to work within the limitations of whatever form of goverment your faction had. A hybrid of this concept might be to have some forms of goverment become permanent once they are chossen (or at least major penalites for changing).

Actually, in SMAC you could alter your government through the Social Engineering screen. But, if certain options had negative effects or positive effects depending on how it realted to your faction's traits. There was a preferred "government" in SMAC.