So, the only question I particularly can think of an answer for is that the Players should be able to influence the plot to a decent extent, cause ripples, destroy relationships, etc. However, if there is a big bad, they shouldn't be able to influence that much outside of maybe making it more difficult for them without realizing it, or making it easier for them without realizing it. 

I'm interested as well. You gonna teach me how to fight, Yoshu-nii?