I think the only possible scenario for Meerab leaving Murtasim will be a misunderstanding regarding Haya. Some people say that Tere Bin drama is ITV inspired so in ITV serials, mostly, the possibility of the female lead leaving male lead is because of the misunderstanding regarding the obsessed girl.
Haya has not been punished until now. So the character is needed in the story. Till now Haya has been the catalyst for Meerasim's love story and now Meerab started realizing her love for Murtasim, so Haya is not needed but she is still there so there is a chance that because of Haya Meerab might leave Murtasim. The director told that Meerab will punish Murtasim. Why will she punish him? Until/unless some misunderstanding happened she won't punish Murtasim especially after contract breach. So I think there is a possiblity that Haya will be the reason of misunderstanding and Meerab leaving Murtasim.
But the writer also said the separation won't be because of any third person. Haya/Rohail won't be able to separate them.
And tbh, post confession and consummation..allowing a third person to come in between makes no sense. I'd rather have her leave because their thinking doesn't match or some kind of conflict/approach.
Maybe it will be something on the lines of going against norm. The setting of Khan Haveli and near by villages in a way has this conservative and orthodox mentality....maybe it'll be a fight between traditions vs adapting to newer times 🤔
I mean the director said Mee will punish MK but will also learn a lesson...maybe this separation will make them realize that traditions are important but are not to be followed blindly at the cost of someone's happiness or freedom or right.