Tagged unions? -- please no... I could be convinced to completely lose unions in C, though (pointer to member or base of struct can be cast to another struct anyway, so losing unions doesn't gain anything; for the same reason tagged unions just would not be useful)

Boolean type? Sure, but that would be dependent on use. What is wrong with a bitfield one bit wide instead? What may be useful is "packed bitfield" type ("packed" in Pascal). Then, array of packed bit could be expressed.

Multiple return types - yes, "return a b;" (or something would be nice.

Lack of polymorphism - reference "void *" The main problem is that calls cannot be constructed in C (that is, the standard does not have a "C to C" FFI.

Anyway, just food for thought.