Fame is not that important to historians. Famous people's lives are generally well documented in sources other than a journal - newspapers, proceedings, parodies, letters, legislative records, etc. PRivate people's lives are, in fact, a lot harder to find out about and lot more interesting to people practicing history today. What we know about events like the Civil War or even World War II, life aboard an American whaleship or on the frontier, we mostly know by the writings of non-famous people who simply observed and recorded the detail of daily life.
At some level it's impossible to say what historians of the future will find important and interesting about our time, and it will change, anyway. Diaries of non-famous people were scoffed at for a long time before they began to be taken seriously as sources of historical evidence. But I think there are some general principles.