Doing a "Stratux" type device using Software Defined Radio it should be possible to create an RNAV box that can determine where it is using the remaining VORs, AM/FM/TV/Cellular/ADS-B/Radar transmitters, etc. Add in the capability to not only use the US GPS system but the ones other countries are launching too and you'd have the ability to find your way in almost any conditions. Edit: oh, yea, can't forget ATIS/AWOS transmissions too.
If only there were some sort of high power long range navigational signal that the government could transmit from various places around the country.