The next issue would be the smallest issue date in this table that would also be greater than today's date.
Set up this relationship:
MainTable::cToday < Issues::IssueDate
and specify that the records in Issues be sorted by IssueDate in ascending order.
Then a reference to Issues::IssueDate from MainTable will refer to the next issue to be produced.
cToday would be the unstored calculation field: Ge t( CurrentDate )
You can also perform a find for all records with IssueDate > Today's date and sort the found set by IssueDate in ascending order. Then the first record in the found set will be the next issue.