Here is a whitepaper about how to define a new proof of work system that gives preference to CPU's forever theoretically and is immune to velnerabilities like encryption algorithms are prone to.
(link is to wayback machine so you can trust visiting it)

https://web.archive.org/web/20171212224738/http://www.naturehackerproducts.com/2017/12/new-proof-of-work-pow-method-based-on.html?m=1

The take home message is to find a single factor (of a length defined by the difficulty) of a very large number (over 100 digits).

Sunce it is such a large number GPU's become very slow at attempting it.