If Luna was to assume full (or even 2/3) power, we would probably end up with Nazi Equestria. She has a highly unfit psychological profile for such a role. Ponies don't stay awake during your night? Destroy all life on the planet, and begin by brutally murdering your sister. She has a troubled, to not say depraved personality. She could probably precipitate on diplomatic affairs and do something extreme. Plus, she has little grasp with the modern world in general, and is probably very inexperienced with the political system. That doesn't make for an efficient government either. Honestly if I was Celestia I would keep her in a limited role, and have the guards keep an eye on her. She definitely didn't come back to power in a trustworthy fashion, and appears to have a pretty low acceptance rate from the general public.