There are now tons of Certification Authorities (CAs) that are trusted by default in major OS's, many of which are unrecognizable without online lookup or reference.

While there have been attempts by the NSA and others to "hack" or otherwise exploit root certicate authorities; is there anything preventing the NSA from becoming a Root CA itself?

It certainly has the resources and expertise, and could "suggest" to major OS vendors to add its Root CA to the default trust store list (which is large enough that it may not be noticed by anyone..?)

If it is feasible, what would the implications be? Could they essentially Man-in-the-Middle attack most HTTPS connections without a warning? (Perhaps not Dragnet-type interception, but close?) Or create a fake commercial root CA as obviously people would be suspicious if it had NSA plastered all over it?