It would be nice to able to create a separate web application with a separate data model, offering whatever functionality is desired such as contacts management, SLA management or whatever.

And then create a custom field in Jira which has a drop down selector tied to a column in an SQL table belonging to that application. So when in Jira you can open a selector and choose one of the records in the external application.

For instance when creating an issue you could select a customer, which lives in a separate application. Or you could choose an SLA agreement.

The Custom field would need to be configured with the SQL table name, the SQL column name containing a unique identified and the SQL column name containing a human readable description of the row. For additional flexibility you may need to configure a database connection so you can connect to an external database.