1. Standard library support for safe arithmetic (easily capturing overflows, underflows, and so forth).

2. Standard library support for threading and synchronization primitives.

3. Mandated tail-call optimization through pragmas or perhaps just requiring it in the spec.

4. Create explicit and differentiated arithmetic and logical shift left and shift right.

5. Add a standard way of specifying arbitrarily-sized integers (with optimization for the common word sizes).

6. Add destructuring operations.

7. Add anonymous unions.

8. Add switching on strings in switch statements.

9. Add proper UTF8 support.

10. Add a slab allocator to the standard library.

11. Add regexes to the standard library (possible as a language feature).

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Several of these (7, for example) are being addressed in C11 (if that ever comes out).