Echo Project had a super awesome idea that only was touched on at the very end of Arches that I immediately thought would make a great TV series.

A detective that literally has his dead friend’s soul inside him that can see and communicate with the dead and extra-dimensional beings assumed to be demons. Just that alone would be awesome, but there is also the angle that the voice of his friend isn’t real and the protagonist is just hallucinating everything, and only coincidentally able to solve mysteries because of the supernatural guidance received.