My guess is that a best plot for season 6 would have been the return of Romano. He would have had a master sinister plan to wipe out all the souls of the light, like Melinda, and use their energies to grow stronger. Another topic would have been Aiden's powers. Mel and Jim would slowly start to realize that their son has more hidden abilities, waiting to be discovered. I mean, these are the first things that come in my mind. What do you think?