Well, there are two options.
First - Tragic route - You can show Nishant reminiscing Piya. But then again you have already written so much on that in Shattered. You can go the filmy route that somehow he'll come across Piya celebrating her birthday alone and feel bad for her. So the father-daughter duo will be together.
Second - It can be after PiyAnsh's wedding. So Ansh would call him and inform that they are gonna throw a surprise birthday party and Nishant would go back to FB, then imagine Divya who might ask him not to be sad and enjoy and he might say something like he missed her and wished she was there. And she would be like, she will always be there, in their heart. And then Saavi coming and breaking his imagination and them going for GH.