I have a hunch that either Sid or Roshni will become unstable mentally now after the supposed death of DD.
I want it to be Sid. It should be Sid. He promised Roshni he would save her mom and bring her back but he failed to. He also witnessed first hand of her falling to her "death". The guilt and shock can possibly make him mentally unstable. Yes that would make Yash win, but then we can see two things. One is the love of Roshni in taking care of Sid and helping him recover from the guilt and shock. We can also have Roshni this time fight for both her mom and her Sid. By making Yash pay for what he has done. Also, he is the Jamai who has vowed to always protect his saasural along with his own. So for him to not be able to save DD, would make him go crazy. I mean the way he was beating Yash up in the OLV, did seem like a lunatic about to attack...lol!!
At the same time instead of Sid, it can be Roshni who becomes mentally ill. Her so called best friend who she blindly trusted decieved her. He traumatized so many of those she loves. Her mom fell to her "death" because of Yash. If Roshni becomes mentally ill then as usual we would see Sid showering his love and care as well as fighting to make sure Yash deserves the punishment for making his saasumom fall to her "death".
Or we could have both become mentally ill. Which would lead Sam of all people (SHOCKER!!) to fight against Yash for all that he has done. She might even punish herself. Realizing that if she was more caring and was not so insecure about all then maybe all this would not have happened.
I want the ill person to be Sid so we can see Roshni taking care of him for once. Also, I know Ravi would do an amazing job as a mentally ill person. He did act as an autistic person in 12/24 Karol Bagh, so he would be awesome. Also, I feel it makes more sense for it to be him.