Something I think would be a fun addition to say a hunter would be an exotic that allow two sidearm like weapons to be equipped in the primary slot.
Two sub-machine guns (less powerful and shorter range auto rifles) for titans
I don't know what for the Warlock, but two short swords would be cool, even though it seems to go better with a hunter.
Gives class uniqueness and adds something new.
I'm sure this has been discussed before. But a friend and I were thinking about it recently and just though, hey, that would be cool.