Insert closing delimiter char(s), i.e. parenthesis, bracket(s), brace(s) or whatever is needed - think of syntactic consequence.
Honor multi-char pairs like triplequoted string, padding and escape-sequences.
For example when called at the end of an open string like
"\(^ *\|^Foo: *\|\( BAR\|'s\|Bad\|BAZ\|Bnter\(?: \(?:\(?:sam\|th
syntactic-close RET should result in
"\(^ *\|^Foo: *\|\( BAR\|'s\|Bad\|BAZ\|Bnter\(?: \(?:\(?:sam\|th\)\)\)\)\)"
That way continuation may be inserted by just one key.
A first draft was proposed at emacs-devel mailing-list: http://lists.gnu.org/archive/html/emacs-devel/2013-09/msg00512.html