Well, what do you want to know about them? What kind of audience is this for? An audience of non-fans are going to want different information than those who are hardcore fans.
For the fighters, you can ask common questions, like why they fight, who their role models are, how they got into MMA, and so on. You could also ask them how they balance MMA with the rest of their life (work, kids, social life). Another question might be how they justify participating in a sport where they look to deliberately hurt another person.
For the coaches, once again, asking them how they got into MMA would be interesting. You could ask them about how they deal with having to juggle several different fighters, or how they deal with different fighters' personalities. Perhaps you could delve into the financial aspect and aks them how they keep their business afloat or what cut, if any, they get from the fighters' purses.
Just some ideas. I'm sure that once you start asking the actual questions, new ones will pop up.