I feel that this is a very challenging question to answer. As someone who has done a lot of studying into the Catholic Church, when a pope dies, the cardinals all gather and elect one man, usually one of them (however there have been cases of external nominations).
The issue I have with the nature of this question is that when you are nominated for pope, you don't run in the same way one would for president or prime minister. You don't create a platform of "This is what I promise you if I am elected!". You may be asked questions by the cardinals on topics such as your values, what you hold as more important, or even on your personal philosophy, but you do not create campaigns.
To say "If I was pope, the first thing I would do is to change this!" is quite improper. You could say "If I was pope, I would gather a council to look at the possibility of married priests," but not something definitive. The pope actually doesn't have unlimited power. The pope has the right to veto certain actions, but only to allow the maintenance of current teachings. An example is the publication of the encyclical letter "Humanae Vitae". A council of people were gathered in order to discuss whether or not the Church would uphold it's teachings on contraception. The majority report (the majority vote) said that the Church would change it's teachings on contraception, allowing it. Pope Paul VI, who was on the side of the minority report, vetoed it and decided to maintain the original teaching. If the next pope decided that he was on the side of married priests, he could not just "legalize it" in the same way that countries would legalize a new drug or service. He would have to order a council and they would have to have the majority report accept allowing married priests. If the majority report chose to uphold the teaching on celibate priests, the pope could not just veto it and change the teachings.
There are other rites in the Catholic Church that do allow for married priesthood, such as the Byzantine Catholics, just not the Roman Catholic rite.