It would be very useful to be able to define column types that have their own set of rules/formulae and to be able use them across several schemas so that changes in the base definition could be propagated to the schemas in which it's used.
For example, I have a date field that had to be modified in base table schema but which appeared in 40 others (error test data).
passportIssueDate ::= [equineFullDoB + days(passportIssueDays), now()].min