Your main character can be anybody: man, woman, child, animal, robot, even a vehicle (Cars). As long as he/she has a goal that’s difficult to achieve and vital to his/her well-being, you have a wealth of options.
The goal has to be “vital” or the stakes won’t be high enough and there’s no reason for the audience to care. It needs to be difficult to achieve because if it’s too easy, there’s no suspense.
There are essentially two ways to make the goal difficult to achieve. One is to make the antagonists that oppose your main character and the obstacles he/she has to overcome as overwhelming as possible. The other way is to make your protagonist seemingly as weak as possible, either physically, mentally, or weakness manifested through internal flaws.
Take The Lord of the Rings. The dangerous mission to destroy the ring into the fires of Mount Doom that ultimately will decide the fate of Middle Earth is entrusted to two Hobbits, the smallest (and weakest) creatures in the land. In comparison to the fearsome foes they will have to evade, the Hobbits are least likely heroes of all the characters in the story. And not only are they physically weak, but Tolkien throws as many external (and internal) obstacles across their path as possible.
So when you’re thinking about your own main character, think about how you can make him/her the wrong person for the job. Make them the underdog. It helps to give them internal flaws that make them ill-suited for the task at hand.
In The Verdict, it’s an alcoholic lawyer who has hit rock bottom that has to take on the Catholic Church and their high priced legal team. His inner demons and the opposing counsel attack him from both sides.
Make it ironic that your character is the hero of the story. Does your character have to dress up like a woman? Then make him a misogynist or a womanizer. There’s a reason “fish out of water” stories always work. The main character is ill-equipped for battle and that keeps the audience in suspense. And this allows for a much larger journey and a more rewarding character arc after the protagonist overcomes the obstacles and accomplishes the objective.
So think about how your main character can be the wrong person for the job. And if they succeed in the end, think about how they become the right one.