Blockchain, cryptocurrencies, or your favorite other term here. I gave my outlook on these to the people at Focus Economics and you can find the post at this link, Blog Link.
These are a polarizing topic in the world of finance right now, so I think it is an interesting topic and question and worth a read. One of the big issues out there right now has to be the issue of regulation of the currency. Several countries already severely limit or outright ban these alternative instruments. Would the US see a need to do this? At some point possibly, at least from a currency perspective. The investment aspects would be the concern right now and they would simply need to make sure the currency is subject to the same regulatory regime as similar investments.
It is not a major threat to US monetary policy at this time, but again, it should be on their radar as a potential future threat.