At the current moment they are making trusts available based on story relevance. The Media tour build had Thancred, Minfilia, Alisaie, Alphinaud and Urianger as available trusts for the lvl 73 dungeon. Y'shtola (Matoya) was not available, and would never be available for that dungeon because story wise she is not there, and we know she can be called upon for other instances. If they make more trusts available, they would be based solely on story availability. This stance in turn, makes it harder for them to retroactively add their version of trusts to older content, because most of the time we canonically go through dungeons w/o any of the npcs beside us. There are very few dungeons where it is otherwise.
