I have a feeling the next generation of U.S. notes will probably end up as a polymer. A friend of mine worked for a while with the Bureau of Engraving and Printing about a decade ago and said that polymer notes were being tested, but at that time, none could survive the very rigorous durability testing the U.S. uses while the linen paper just barely managed to.

But we're in to second or third generation of polymer notes now and the chemistry has improved to the point where they'll make it through the ringer. However, like this article points out, banknote R&D has many years of work behind each issuance, the new U.S. notes that are just now coming out started development 10-15 years ago and the new ones use a new paper formulation. Bonus, the U.S. gets the benefit of observing the results of field usage for other nation's banknotes before committing to a couple trillion dollars in cash printing (more than any other nation).

Here's a brief video on the evolution of the U.S. Banknote https://www.youtube.com/watch?v=QjLD2B5I9yk

https://www.youtube.com/watch?v=2DaVsmsHq0E

Here's some videos on the type of durability testing from Canada's new bills

https://www.youtube.com/watch?v=8odUVIAXPS8

https://www.youtube.com/watch?v=-lDHLNgtzao

and an example of where they fail the test https://www.youtube.com/watch?v=znfOzuCBpAc