No offense intended but this sounds like Core Feature Creep, which IMHO is a slippery slope and better avoided. (I am admittedly biased having seen WordPress forge ahead bloating itself by adding “more” at the neglect of fixing the existing in an apparent desire to become “everything to everyone”)
That said, I would have no problem with a “custom field” being added that plugins could use if none of the currently existing tables provide one.
AFAIK, there are custom fields for
- category
- group
- post
- topic
- user
Maybe another eg. notification_custom_fields would be a better idea that adding limited use fields to tables?