This is a really interesting question. The idea of intentionally engineering a "useful primary source" is in itself kind of difficult. First off, make sure that you write in your journal somewhere that you had that in mind when writing things down. One thing that always interests historians is the motives of the people who have written their sources and when you choose to write things down, you're making a selection of sorts about what gets passed down to the historian who ultimately finds and uses your journal. Letting them in on the plan, so to speak, will give them valuable information in itself.

The most important thing about journals as primary sources is that they are one of the few ways that historians have of getting information about what people were thinking and what their individual experiences were like. In this sense, I would try to avoid engineering a narrative so much as I would try to as honestly as possible write down what you are thinking about what is happening around you, how you feel about it, what kinds of conversations you have with friends and family, etc.

Another way of looking at it is this: In the future, historians will likely have access to archival and other material that gives them a great deal more insight into the politics of our present moment than we do currently. This is particularly true for those of us without any kind of insider knowledge about what's going on. However, all of those documents in the world aren't going to give the historian information about what is was like to live through this period. Chronicling the front page of the Washington Post isn't going to do historians much good either, because they're going to have that available to them. Of course, what you think about the front page of the Washington Post is something that is otherwise completely inaccessible to my future colleagues.

Don't try to be the historian yourself. Just try to approach writing your journal in a way that feels intellectually and emotionally honest. It's you that future historians are going to be interested in and your thoughts, ideas, opinions and reactions that are going to be quite literally only available to historians by what you've left behind for them.

What kinds of things historians of the future will be interested in when looking back at 2017 is hard to predict, as are the frameworks through which they will be doing so. What kinds of questions will they be asking? How will the huge amount of data available from social media influence the profession over the long term (assuming it is successfully preserved)? We'll have to let the historians of the future decide what sources make sense to them. If you try to engineer your journal to what you perceive as important now, you might leave things out that seem critical in the future. What you can do is give them your completely unique perspective that they'll be able to use however is appropriate after we're no longer around to see how.

1. Be yourself. Historians are trained to identify and work with the biases of their sources and often knowing the author's point of view contributes greatly to the story.
2. Focus on making your perspective clear. Make your news sources clear and your responses to them explicit. When I read diaries from the nineteenth century I'm trying to understand how events affected people socially, emotionally, and politically.
3. Talk about the relationships that affect you. Friends who you talk about stuff with and your proverbial racist uncle who is always posting fake news. Bare bones social networks will probably be easy to build with a facebook archive, but knowing what someone thought privately about their social network will be tougher. This is especially true for those times when you tune out.
4. Don't worry as much about collecting mass produced stuff, but do worry about collecting ephemeral stuff. The Time cover is not as important as the weird white supremacist literature getting shoved in your mailbox or the flyer for a punk against trump concert. A good rule of thumb is that it won't get archived if it was sold in an airport bookstore.
5. Have fun, and don't feel bad if you abandon the project after a couple of months or a year. Most of the diaries that I've found in archives have similar gaps, but they're still interesting.

An interesting idea but remember that every action and editing you do will instill biases in your source because all primary sources have biases. So it's hard to say what makes a primary source "useful" since it depends on what the future historian is looking for. By consciously telling yourself you're only going to put in what you decide is "useful" for posterity, you've now created a document that's very problematic. Of course, it could still be useful fifty years from now if someone wanted to know what a person your age thought was "useful" in 2017. Or, oftentimes we would want to know what you DIDN'T think was "important" or too "mundane" to mention at length.

I definitely recently became interested in doing the same. Except instead of doing a journal (my penmanship is less than adequate), I'm instead collecting artifacts of today. For example, I just picked up TIME's recent article which portrays the White House as being converted into Saint Basil's Cathedral in Russia, due to the allegations of President Trump being tied to Russia. I'll also probably start collecting more newspapers for big events. This sparked my curiosity when I found a newspaper from 9/11 in my grandparents' garage. I digress, I apologize. Anyways, here are some pretty historical recent events that I think will be talked about in the future-

The election of the 2016 and all the controversies that surrounded it. The countless demonstrations and protests. Terrorist attacks/the rise of ISIS (the one on London yesterday, Ariana Grande concert, etc.) The Berkeley riots. Black Lives Matter and the violence that has occurred during those protests. The Paris Climate Agreement Kathy Griffin? (To show the amount of hate that's going towards our president) This is all I could think of right now. Hope this helps!