I also like the Ken Thompson extensions from which part of Go was inspired by

    struct X { int a; }
    struct Y { *X; int b; int c;}

    void add(Y* self, int number) {
      self->a += number;
    }

    Y y;
    y.a = 10; // composition
    y.add(1) // y.a = 11 now

This alone would simplify C coding so much without taking any power out of it.

The other extension i would add is some sort of interface or protocol.

As soon as you can do something like "y.add(1)", having a generic contract to refer to things without having to know its concrete type is some of the good things from the OOP world.

With this you would also be able to call some cleanup code and even a initializer.

This is still C and its still much simpler than C++, and yet almost as powerful.

C should propose these kind of things even if it was not that conservative and it would retain a lot of coders that migrate instead giving C barely evolved from its 70's roots.