Thank you for mentioning Unknown Unknowns explicitly. I suspect that they are one of the least discussed, simply because of the difficulties involved. However, I do think they may perhaps be more tractable than they appear at first. For example, there is a substantial existential risk posed by global catastrophic risks from which civilization never fully recovers. (This GiveWell article sums things up nicely: http://blog.givewell.org/2015/08/13/the-long-term-significance-of-reducing-global-catastrophic-risks/ ) I think trying to increase the probability that we recover from a broad range of GCR's could make a significant decrease in total existential risk.

If we expect GCR's to be much more likely than X-risk events, and if there is a reasonable chance that our civilization will never recover from any given GCR thus turning it into an X-risk, then it seams prudent to put a lot of effort into increasing the probability that we recover from various forms of collapse. Existing efforts include things like seed banks, fallout shelters, and efforts against a digital dark-age. However, those projects seem to be done for their own sake, rather than to reduce X-risk. I think it is likely that they would do things slightly differently if X-risk was the terminal goal. This implies that the counterfactual good we might do by joining may well be large.

Note that this also implies that work in the humanities might be especially useful, if it explores how and why civilizational progress occurs. Understanding the mechanisms might allow us to better predict what sorts of disruptions cause collapses, and what factors are most important in a recovery. Perhaps history, sociology, anthropology, etc. would be good career paths for people interested in reducing X-risk. (at least if they think they can help promote relevant academic research within their field.) We tend discuss the more quantitative fields here, perhaps overly so.