RDF is becoming more and more important as a means to represent information that overlaps with TEI's domain, and it is likely that the representation of RDF-style relations in TEI will become correspondingly more important. RDF data is a directed graph of relations, where each triple represents one arc between two nodes (or between one node and some data).
The set of attributes for denoting the ends of the relation arc are @active, @mutual, and @passive. Because the relation element represents one or more arcs in a directed graph, I suggest TEI use the same attributes it uses to indicate the ends of an arc in its own vocabulary for representing graphs: @from and @to. These attributes have the advantage of being unambiguous, unlike the present ones, where it is easily possible for an encoder to become confused about which attribute to use for which end of the arc. This would also normalize the behavior of relation and arc, which are doing essentially the same thing in different domains.
My suggestion would be to add @to and @from to relation and to deprecate @active (@from) and @passive (@to).