With all due respect to the 538 crowd, it is far too early to tell.
1. Who really knows who's going to get the GOP nod? That could have a significant impact on what issues are focused on in what ways (or whether the biggest issue is the candidate themselves).
2. How about the Dem candidate? Almost certainly Hillary, but who knows where she may need to tack toward given Sanders counter-campaigning? And, of course, Hillary is herself an issue.
3. What's going to happen over the next 10 months or so? Another tanking economy would have an impact. A major terror attack on US soil would have an impact. Revolutions, scandals, military reverses, Putin, China, gaffes in debates, etc., could all impact the electorate in ways that can't be predicted now.
Americans, unfortunately, have short attention spans (e.g, http://goo.gl/xH4koD). While a third will vote for their party regardess of the yahoo nominated, there's enough potential swing there based on the course of events to make figuring out what we'll be talking about as the big issues in September 2016 90% speculation at this point.
The Big Issues Of The 2016 Campaign
With less than a year to go before the election, FiveThirtyEight’s staff members took a detailed look at seven issues we cover regularly, examining the positions of the presidential candidates and …
I would like to state that the biggest issue is that I have to hear about this now.
Does it really matter? Those of us with Sight already know……
Chelsea 2024!!!