Just like what happened on the ending of Code Geass : Lelouch of the Rebellion, the story has ended already. Death Note’s main characters are already dead. Light and L are already dead that’s why there is no other reason to bring up another third season.
Yes, I know you are pretty much disappointed but we have to give up on things. Good things always end and there is always an end on everything. Everyone’s life has an end, and so our favorite animes.
But if you are still on the hook and wanted to reach and feel more about L and Light then feel free to read its light novels around the internet. I think the two light novels entitled, Another Note: The Los Angeles BB Murder Cases, and L: Change The World are two great novels that would really fit your reading list.