Was just thinking about this while browsing wowpedia and I have three possible candidates/ideas for who the shard might be.
-It's a piece of Anduin
-It's what's left of Varian after he got blasted into dust by Gul'dan
-It's the piece of Arthas Sylvanas saw after she threw herself off the top of Icecrown
Personally I think 2 and 3 are most likely but I could be wrong and the first one could also happen.