One thing I've always thought would be useful is polymorphic column types, to reduce the need in "generic" records of having stuff like
create table foo (id int, b varchar (list of typenames), c1 type1, c2 type2, c3 type3, ...);
A polymorphic column type would be a column that can take on a type specified in another column, so the above would be replaced by
create table foo (id int, b typename, c typespec(b));
where column "b" specifies a type, and "typename" is a special pseudo-type that would contain names like "int", "varchar(10)", etc.
Polymorphic column types would behave like variable-length columns for storage purposes, and would probably only be retrievable as opposed to searchable, although we could allow them to be searchable if we insist that the search operator/function be type-compatible with the row and evaluate to NULL if it isn't.
The client-side row fetch API would need to be somewhat clever to handle this, but since the type of a given row is well-defined, this shouldn't be all that hard to implement.