I would not take much account of transits in this situation in the sense of their reflecting the future of a relationship, except for an exact aspect between a transiting outer planet, especially Pluto, and a natal planet, especially Mars, Venus, Moon, or a planet in the seventh house.
What I would look at more is aspects between the two natal charts. Again, it is the outer planets, especially Pluto, which act on the other chart in such a way that it makes the subject of that second chart experience the relationship as fated in some way. Neptune can also be important in this respect, in his "unity" mode. However, the danger here is that he can also represent illusion with its inevitable subsequent disillusion.
Also, the dynamics between the natal planets of the individuals tell us how they perceive each other and interact. However, you might also want to look at the composite chart, which, in effect, can be seen as being the chart of the relationship itself seen as an entity.
Obviously, insights from the composite have to be merged with those from the interaction of the two natal charts, and also with what is learnt from each individual chart in order to get a full picture.
krys