Hood-up robes for Sith and hood-down robes for Jedi
Particularly Malgus' hood-up variant of the Sith Champion set. And Satele's outfit for Consulars would be amazing too! Even if it was a reskin so that Satele's version remains unique.
Would be quite nifty to have the Sith Champion, Vendetta, Battlelord and Force Magister sets available for a wider range of classes but I understand the appeal of having "class-specific" sets too.
There are a whole bunch of social outfits that never made it in the game that would be great to see too, they can be found in a link in this thread: http://www.swtor.com/community/showthread.php?t=549418