We can already reacquire exotics we've obtained before, so why not do the same with unique legendary items we get from quests? This already exists in the game to some degree (after doing the sword quest we can buy the other two legendary swords from Shaxx, same with the Refer a Friend sword with the Speaker). I believe things like class exclusive legendary weapons (Gunsmith Rank 2 quest), the class items you get at the end of each "Path of the X" questlines, and (possibly) the raid gear you've already obtained should be considered for this. I mean, we can already do this with exotics and the swords, so why not the other stuff?