Its probably more interesting to transport the actual players into the D&D world. They would mostly start at 1st level, with artistic license applied at to what class. The magic that brought them across worlds would also transform their abilities enough to become 1st level in a class related to their natures.
If a player had a purple belt in aikido, the could be a monk. A graduate student could be a wizard, a person who had served in the military could be a fighter, etc.
Intuitive types might become oracles. A a chemistry student, or wine lover could become an alchemist, etc..
Making extraordinary people from our world (special forces people, MMA fighters, etc.) is less interesting to me.