The most important figure of the country is definitely the ruler of the country. The ruler plays a very important role to foresee the countries success. There are many key decisions to make on where the reserves of the country should be spent on and where these cash would be spend on. If done right, It usually has this multiplier effect which increases a countries wealth exponentially.
Every single dollar spent in a country no matter by what means would increase by five to six folds. Imagine just spending a dollar by the government to the economy, it would usually become usd 6. This is a very powerful concept that everyone has to have which they have to bear in mind.
The second way is make sure that the people are well educated as education plays an important role in the way the country or nation progresses. A country with more educated people would have a more advanced society. Education is the key. It is arming the next generation with the required knowledge that is going to be used in all aspects. The new knowledge would be the increase in technical areas where electronics and a. I is concerned. Countries with less experts in this areas would be left behind as they can't built advance technologies.
Debt situation - how much debt a country has is important in making the country progress. The more debt a country has, the harder it is to progress as a lot of the wealth that the country earns is usually spent on repaying its debt
If you ask me who, I would have to say the people. Leaders go corrupt or are pressured to do corrupt actions. So yeah, power tot the people or something like that.
And how you improve the nation, by putting more weight on the choices of the people. Put it like this, if you are in a group with 100 people and your voice counts as one, but the voice of others counts as 10, you would feel powerless, and not very confortable or appreciated in the group. That is what a politicians are, people with more weight on their voice, so other people who speak up don't feel as part of the group just because their voice in't heard. So to fix that problem, you have to give everyone the same weight to their opinions, that will make people feel more included and believe more in the group, or in this case, the nation.
Unless the nation can elect a leader who is willing to face the truth.
If I get your question right, you are asking two questions in one about who can make the fate of a nation to improve and how can he or she make it to improve.
Firstly,I will say a great and good leader ordain by God is only the one that can improve the fate of a nation. A leader who is not selfish. A leader who is also concerned about how his or her citizens life is and not only concerned about his or her own selfish gain. A leader who is not corrupt. When a leader is corrupt, it will be 85% sure that the nation will also corrupt and in this case, the fate of the nation can not improve. A leader who will be capable to take situation that will both impact the nation positively.
With these attributes, he or she will be able to improve the fate of a nation.
Now to the second question, how can it be improve. By the nation first believing in God as only Him can improve the fate of a nation. By the citizens of the nation also deciding in their heart to work together so as to improve their nation fate. And lastly by the leader been able to take good and correct decision that will impact the nation positively and not negatively.
With these that I have mentioned, the fate of a nation can be improved.