What was the TV series that failed do you think had the potential to be great? I'll say 2 season max. I know there will be a lot to say Firefly so beside that.
Mine is without a doubt a show called Raines. Jeff Goldblum played a detective that would get into the mines of the victims of his cases. So much so he would see them as he solved and imagine having conversations with them. It was an actual original idea IMO. Not a ghost but just a mentally psychotic person that could really get into the minds of victims. Really try to find it on line and watch the Child predictor episode. You will cry I don't care who you are. This would have been a touching series that explore the dark side. It was done well I don't know why they didn't give it a real shot.
I will also give mention to the shows The Dresden Files and Whoops. Both had kinda stupid stories but had great comedy value. The Dresden Files was basically imagine Harry Potter grown up and is an alcoholic detective who killed his uncle. Whoops was total BS but was funny as heck. I can't even describe it you just had to see it. It was just a show made to be bad. It was great.