The DiMeras, most likely. They could say RealWill was taken, hidden away and switched with the FauxWill we saw murdered by Ben. Some are suggesting they could say it was why Will became more evil (like Sami) while Guy Wilson was playing him.
If it's true, I'm a little surprised Chandler Massey would return. He seemed so bored with the character when he left. It was obvious in his performance towards the end. Time is the great healer, I guess.