When I am writing original cases, I like to work backward. I start by asking myself what I need this case to accomplish in the story. How will it affect the characters? How will it move the plot forward? Once I know what I need it to do, I decide what kind of case it will be. Different crimes affect people differently, so I determine whether a serial killer, arsonist, or rapist is what I need in order to make this accomplish its goal for the plot.
After that, I will research the profiles of this type of Unsub. The criminal minds wiki is very helpful, but I do a lot of my own research and googling. Once I have rough idea of how the profile should look, I will build the case. I try to add something unique, but sometimes keeping things simple and standard is the way to go.