I work in 19th century documents. My dream journal, diary, or letter would be written in pen, with clear handwriting, on a decent heavy paper. The ones I dread having to sort out are written in pencil, on thin onion skin paper (the federal government loved that stuff in the 1890s), or with absolutely horrific hand writing (if you want people to read your writing in 100 years, penmanship matters kids).

I think the question of digital copies is super interesting. I think the digital age is absolutely decreasing how much we rely on hard copies of things. There are some efforts to preserve online communication. The Library of Congress is backing up some websites related to specific themes like elections. The Internet Archive has a bunch of other websites.

Though I don't think that emails are an exact substitute for letters or blogs for diaries. Emails are kept behind passwords and might not be archived in the same way that letters were tossed in a file (maybe the NSA reads/archives them--go nuts boys, my emails are boring as hell). Access to people's personal and professional correspondence might become much harder to get. Blogs could be similar. They are written and sent out into the internet ether. If the writer wants to conceal his or her identity, it's not particularly hard to do and for social historians, knowing who is writing something is important (is it the voice of the middle-class or the poor? A woman's voice? A person of color? A child?). There are ways to sort it out from the writing, but a diary will often, though certainly not always, come with some sort of provenance that offers more concrete information about the identity of the writer. There are also a lot of password protected blogs, discussion forums, and social media sites. I think that it's easier for people to keep private things private and it will change what kinds of research historians can do.