What about the option for resetting the default character
equivalence sets and rebuilding them from zero? A variable
that would allow me to write something like

  (setq char-fold-user-list '((?'  "‘" "’")
                              (?\" "“" "”")))

to have only quotes be considered equivalent. (When
‘char-fold-user-table’ is non-nil it overrides the
default table.)

Do you think it’s a good idea?