In addition to the NDBs, bring back navigation with the AM broadcast stations. Plot the important stations[1] on the charts again, along with MF broadcast airways, much the same as how the old NDB LF/MF airways were charted.
Would be kind of fun to have a WCCO 12L instrument approach into Minneapolis/St Paul Int'l as an NDB-equivalent approach.
With one or two 50kW stations, could start that approach hundreds of miles away.
ADF nav with FM broadcast stations? Sure, why not? (would need equipment that doesn't exist today).
[1] Clear channel, high power, and low power that are well located.