We probably won't know this decade what was scripted for this Leia-centric movie, but here's
why it won't happen, along with apnea as the official cause of death:
Fisher’s toxicology review found evidence of cocaine, methadone, MDMA (better known as ecstasy), alcohol and opiates. The test results “suggests there was an exposure to heroin, but that the dose and time of exposure cannot be pinpointed.”
You can read the 24 page report. Should have spotted the clue in Bright Lights when she was getting in shape for Force Awakens and sneaking some Coke.