In The Hunger Games, who, other than the main character, makes choices that change the world of Panem?

That depends on if you are talking about the first book of the series or the series as a whole, so I will attempt to answer both.

In The Hunger Gamesthe book, one very big decision that sets in motion many other things is the one made by the Head Gamemaker, Seneca Crane. At the end of the Games, Katniss and Peeta are the only two tributes left standing; they had been told earlier...

That depends on if you are talking about the first book of the series or the series as a whole, so I will attempt to answer both.

In The Hunger Games the book, one very big decision that sets in motion many other things is the one made by the Head Gamemaker, Seneca Crane. At the end of the Games, Katniss and Peeta are the only two tributes left standing; they had been told earlier in the Game that if they were the last ones standing, they could both win, but at the end, that particular rule was revoked, meaning that one of them had to kill the other. However, when Katniss and Peeta chose to eat poisoned berries to kill themselves rather than one of them killing the other, Crane stopped them and let them both win. Not only did this lead to his own demise (by the very same poisoned berries that Katniss and Peeta had been about to eat), but it showed to the country that the system could be beat. The act of defiance by Katniss and Peeta (both of them dying would have left the Games without a Victor at all that year, which would have caused problems in and of itself) showed the rest of the country that the Capitol was not invincible, that it could be beat, which helped set off the rebellion.

Within the series as a whole, there are a few others who make important decisions: President Coin of District 13, who leads the rebellion; Cinna (Katniss' personal stylist), who makes dresses for Katniss that enforce her role as the Mockingjay, which enforces the people's decision to support her; President Snow of the Capitol, who tried to suppress the rebellion and ended up only making it worse; the tributes in Catching Fire who help Katniss and Peeta survive and get to District 13; and Plutarch Heavensbee, who ran the Games in Catching Fire and helped the rebels fight against the Capitol.