OOP in Pure C

You're basically asking how far can C-jank be exploited to do something useful. There are so many stupid things in the language, if you combine some of them they actually become useful. You can recreate oop in C, and actually make it more useful see this for an example. In C, you can implement a runtime garbage collector. You could also have closures and even lazy data and lazy evaluation. You can kinda re-implement generics as you've said. And those are just the stuff that pops in my head, I'm sure you can figure out or find examples of other interesting stuff. Some of these work good, others not so much.