I donno, from a technical standpoint it never seemed to me like WannaCry was extremely complicated. I actually described a Ransomware with a very similar crypto scheme on here not that long ago [0], and I don't have tons of crypto experience past taking a class on it in college. With that said I'm sure there are other ransomwares out there that already do something very similar (or even the same).

The more complicated part was the propagation via the exploit, but at the same time the exploit was already out in the open. That was part of the reason it only infected old or non-updated machines. So still not easy, but a good chunk of the initial work was done for them.

[0] https://news.ycombinator.com/item?id=13147605