I saw this article and it's not a bad and I would propose something similiar.
Give Military Units right to rebel and attack a region of their own country. Military Unit can't takeover country if at least 50% of country regions are not under their control (if that country have more regions). When they takeover 50% regions of country, they have right to attack presidency and if they won they takeover the country so Military Unit Leader becomes a new President and other members od Military Unit take place in Congress.
I hope you will consider this proposal, it would interesting.