The widely held belief is that Brock Lesnar would be immediately booked into a rematch against Alistair Overeem if he decides to return to UFC. Overeem was the person that defeated him for the heavyweight championship before he decided to retire from MMA and return to WWE.
Mike Knox could be the next person headed out the door in TNA. While the company likes him, they simply do not have anything planned for him since the split of Aces and Eights. There is a possibility of him being brought back into the fold at a later time.