Two questions:
1. If rebels take over your nation, is it game over?
2. How do you become a Great Power, or to a lesser extent how do you improve your nations standing in the world.
If rebels take over your nation, your government type will change and you will lose prestige. depending on the rebel types, this may be a good or a bad thing.
Becoming a great power entails bringing the sum of your military prestige and industry scores to be one of the top 8 in the world. increase industry by building and staffing factories, army by building capital ships, raising more troops, and upgrading your equipment. Increase prestige by unlocking prestige techs (culture), winning wars, and unocking african colonial land. Also converting colonies to states.