If this handled on schema level, yes, I think dgraph will need behaviour change on processing predicates, that is why I propose to handle this on query level, by introducing new filter like @singleObject or @unary to tell dgraph to return single object instead array for one-to-one/unary relationship.
Another reason why I prefer to introduce new filter rather than using existing “first” or “last” filter is compability, I mean if we change existing “first” or “last” filter behaviour, old dgraph user could be affected, but if we introduce new filter, they won’t affected…