I know there re already a dozen threads about it but,it is quite different.As it is an Ekta Kapoor Serial i think these are the possiblities
1)Nandini will leave Karan and then the leap will happen and they will meet again after the leap.
2)Nandini will ask Karan to choose between Tanya and herself.Karan will for sure choose Nandini and Tanya will turn into a witch!
3)Nandini will say that she will be here only until she proves that Karan ins innocent and when this proves that he is innocent Nandini and Karan meets again.
4)Nandini will go to coma(i don't think this is possiblity..jana hota tou pehlay hi jati na)and Karan will be safe and Karan and Tanya lives together and Nandini comes back from the coma after leap(too long?)
5)What else?