Was There an Epidemic?

If you have a death date for an ancestor and no information on cause of death, have you looked at newspapers for the era? Don't look just for an obituary or a death notice, but instead look for any mention of disease outbreaks, epidemics, etc. during the time your ancestor died. 

It does not mean that your ancestor died due to the disease or outbreak mentioned, but it can be a potential clue.

And if several of your relatives died with a week or so of each other, then it's possible that the illness mentioned in the newspaper was related to their cause of death.