Since we now have the option to do different selection rules on source tables mapped the same target table I would like to see an option to control if the data from a specific table should do an upsert (default) or only inserts. I can see this being used in scenarios when there is an ERP upgrade and you want to retain the mapping from the old system but only as an insert. Another scenario is when utilizing pre-populate from fact for dimensions. Then you would have a table in staging with only the BK and SK and that would just be allowed to do inserts, the other fields would then be updated by the "main" source of the dimension when it's available.
Mapping table option - insert or upsert