Stone would probably be an important figure in the investigation if any of his bragging and wild-eyed claims were true. The chances of that are about the same as the chances that Keanu Reeves will play King Lear with the RSC.
In a way, Stone is kind of an idiot savant. He has made himself the McGuffin! At one time or another, he has probably claimed to be wielding more influence that the Illuminati, so all of the things he has told people need to be investigated. All of his silly claims to have important contacts and influence have been an effective red herring, distracting Mueller’s investigators when they could have been following real leads instead of learning that Stone did indeed claim a direct line to Putin or Assange or Guccifer, but was actually just lying to make himself seem more important.