With transmogrification coming out, a lot of us will be trying to find each legendary at least once to complete our library of item skins. Unfortunately, there's no in-game way to keep track of what legendaries you've yet to find.
I feel like Diablo could really play up the "collect 'em all" drive in a lot of us in order to motivate us to grind for more legendary drops. To achieve this, all we would need is an in-game library that has a list of all available legendary drops, possibly found under the Quest and Achievements button on the UI (or as a new use for the Book of Cain?). Legendaries you haven't found yet could have a question mark on their icon, their list item greyed out, their text missing, etc. A mouse-over of previously found items could show the item's standard affixes as well as what it could roll, effectively a built-in version of what's currently viewable in the online game guide.
As mentioned before, this would motivate players to play in order to complete their library of legendaries. Furthermore, players would become more familiar with what legendaries are out there, giving them drops to hope for.
Just an idea I had.. Discuss away!