Define a darker theme. I think it has been pointed out that TOS had some darker themed episodes. Really its about a willingness to write an episode that is not afraid to hit at core issues, things that your argue sides with your friends after the show airs. Its not just about sex and violence. Star Trek has made us think in the past. I would love to see some "real" characters, people that can find a bright side in a dark world. Who when everyone else is running away they are running in, when all is lost then rise up as a beacon of hope. In a lot of ways the Enterprise and her crew symbolize the best possibility of humanity. I don't want to cheapen it for the "sake" of grit. Rather I would like to see how "normal" people rise to do something great. Face difficult situations in which technology can't bail them out.