I partially agree with Axle as to what a calling actually is. I don't believe a person is "destined" to do any one thing, just as I don't believe a person is destined to be with a certain spouse or soulmate. However, in both cases, I would say that there are careers or people that are great fits for a person. As such, I would leave the "calling" part out of a calling and emphasize actively choosing a fitting life path.
I would say my optimal career path would involve something in the medical science field. Ideally it would be as a biomedical engineer, but I've considered careers as a surgeon, neurologist, psychiatrist, emergency physician, or even sports trainer. I've looked into this area of science because I want to help people, and I believe these professions are the way I can make the best possible contribution to the world. Joining the military has once or twice crossed my mind, but I know I don't have the nerve for that. I've been considering becoming a cop, but I still think I could do a better job helping people as a scientist or a doctor. That's what I've wanted to do my whole (albeit short) life.
I don't believe that everyone has a talent that makes them unique. I don't believe that everyone has an innate talent, either. Natural talent is overrated and raw talent alone will not land you in a successful career (or niche if you believe your "calling" is not career-related). You may have an affinity for anatomy, but if you want to be a doctor, you have to study. You might be able to shoot a gun well, but if you want to be a cop, you have to learn the law. You may be good at dealing with people, but if you want to be a social justice worker, you have to put some effort into talking to people and making picket signs and what have you. My point is, unless you foster your passion, your calling, you won't get anywhere. Some people choose not to do so and as such will never find their niche. That's up to them.
Ventus, this question reminds me of the thread you made that asked if you'd ever want to give up a talent. I don't think most people feel disdain for their assets because they are generally valuable. And if they are, most people will try to strengthen another area or just ignore the talent altogether. I can't say I've ever been unhappy about my love for science and ability to excel in that area.