A series that I wrapped up not too long ago was Ghost Whisperer. Probably the biggest series I was really into for a long time. It ended with five seasons, and while the ending was satisfying by answering questions that were left on loose ends and made viewers feel content, it still felt like it could have continued on for one more season. It was actually canceled on its fifth season, and ABC didn't renew it for another one. Writers and actors were said to not sign any contracts as it was possible another network would pick it up, but that unfortunately never happened.
A lot more story could have been told, and it stands true seeing as that a sixth season was nearly taken up. So, I would see this as deserving.
My other idea though would to have the series renewed, with Melinda's son, Aiden more grown up (college student perhaps). It can elaborate more on the different abilities he has compared to his mother. As well as introduce a new method of story to the series. Then again, I guess that's what FanFics are for.