A very possible theory. It makes sense to say that Diz (Ansem's shell) wants to take his heart back from Riku. But remember in R/R, Diz explains to Riku that he can control the darkness and use it to his advantage, which would make Ansem stronger. Also, Diz made his first appearance (in red) in Twilight Town, so maybe he can only be there because he is "a mere shell". At the end of R/R, Diz tells Riku to choose the path to light or the path to darkness. If he took the path to light, Ansem would be released from Riku's heart, and Ansem would return. If he took the path to darkness, Ansem would grow stronger through Riku and would return either way. Maybe that's why Diz wasn't so happy about Riku choosing the path to the twilight.