I see allot of people discussing Sultan entering Madhu's life as a potential lover. I think this idea appeals to many people because they want justice after what RK did to Madhubala. Some say the best form of punishment will be his being denied Madhubala for the rest of his life. Unfortunatley, I think the chances of this happening are very slim. As per the articles, it is stated that Sultan will develop feelings for Madhubala, they say nothing about Madhu reciprocating his love. Even if she does end up developing a certain tenderness towards him, I still believe that RK will be her first and only love. She can't forget him and she can't stop loving him - no matter how hard she tries. The show is about "ek ishq" - "ek junoon" and RK is and will remain the singular hero in in Madhubala's life.
As an antogonist I think Sultan's role is to create a rift between Madhubala and RK. After falling for Madhu, RK will be his most immediate competition. Eventually though, he will probably be a means of bringing Madhu and RK closer, by inducing in him feelings of jealousy and possessivness. It may make RK regret what he has done to Madhu after seeing her being loved by another man.