That is a question I have never asked of myself nor have ever thought of answering. What it will ultimately be, if there is one at all, will be for others to decide. So it is perhaps, presumptious of me to answer, but being one to not let a challenge pass by, I'll make a stab at it.
I once defined a photograph as being a "rhythmic event." At least that is what my photographs are to me. It is my hope that the rhythms in my photographs coincide with life rhythms, much in the same way that great music does. If they do, they have a chance of adding to the art of the centuries that moves us.
In his time, J.S. Bach was considered a great organist, but not much of a composer, since Baroque music had already been done. Later we found that he had made a great contribution indeed. Similarly, traditional, straight photography has already been done. I would hope that the rhythms in my photographs take it beyond what has already been done. Do they? As I said above, that will be for others to decide.
The legacy any photographers work has is partly a function of how visible it is. Think of Disfarmer or Heber Springs. No legacy at all, until someone discovered this small town photographer's portraits and published a book of them. Is it important to leave a legacy? Not really. What is important is to do the work. Do Paula and I work at getting our photographs "out there"? We sure do. But it is not for the sake of a leaving a legacy--for us it is a matter of survival and of being able to continue making photographs.