Create a space not just for improving this site, but for supplying suggestions to different agencies for improvements.
I just read about the revised $100 bill to combat the superbill. However, this is a technology cat & mouse game with another vary sophisticated organization.
One improvement that might be more difficult to overcome is to insert a field programmable RFID chip into each bill. The chip holds two fields - the bill's serial number and a unique number assigned by the US Treasury upon each transaction (transaction ID). Both fields would be encrypted together into one 96 bit field (corresponding to the new RFID standard)
As bills are used in transactions, they run through a scanner which validates that the transaction ID on the chip is valid for that bill by asking the US Treasury for approval, much like a credit card transaction. If not valid, the merchant or bank would know immediately. If valid, the merchant gets an approval code and the bill is re-programmed with a new transaction ID for its next use.
Because new bills issued by the Gov't would have encrypted data North Korea, someone in South America, or whom ever, would be unable to produce new bills because they would have no clue on what # to put into the bill's RFID.