One option is to unify this proposal with the RnRS directive syntax as
follows:

- If #! is followed immediately by a horizontal whitespace character,
read datums until end-of-line. If exactly one datum was read, and it's
either an empty list or a pair, return it. Otherwise wrap any and all
datums in an implicit list and return that list.

- If #! is followed immediately by a newline or end-of-file, return '().
This special case is a logical consequence of the above general case.

- If #! is followed by something else, read one datum. If it's either an
empty list or a pair, return it. Otherwise wrap it in an implicit list
and return that list.

Hence we would get:

#!r6rs                       => 'r6rs
#! r6rs                      => '(r6rs)
#!(r6rs)                     => '(r6rs)
#! (r6rs)                    => '(r6rs)
#!#(1 2 3)                   => '(#(1 2 3))
#! #(1 2 3)                  => '(#(1 2 3))
#!no-fold-case               => '(no-fold-case)
#! foo bar baz               => '(foo bar baz)
#!foo bar                    => '(foo) directive and `bar` ordinary code

#!(declare-things
     (multi-line things))     => '(declare-things (multi-line things))

#! (declare-things
      (multi-line things))    => '(declare-things (multi-line things))