No, the Death Note was not about Shinigami, but where is the line between good and evil, it was about people, conflict… The authors explored that and told that story perfectly for the most part. There is nothing left to tell. Death Note and Shinigami were just an interesting and original tool to write all that and everything that you needed to know to understand the story was explained.
Manga ends a bit differently, there an epilogue. Years after manga ended they did a few one-shots showing some events after, but they werent that good, so if they did continue it, then it would be pretty bad.