A) Less screen time (should Mrunal wish to film for both shows or if her contract permits it)
B) Recast (who would you like to see in the role?)
C) Killed off (By whom? For what reason? In what way: martyr, illness or senseless tragedy?)
D) Goes missing (how and why?)
E) Written off in a way that she can one day return (elaborate)
I'm going with "D" she 'goes missing'...only for her to return one day with Papa Arora, because I've never ever believed that man to be dead. The Arora women never speak of him - which leads me to believe that it's a sore topic in their household. I haven't thought this through. I don't know how, what or why would cause Bulbul to disappear or why she would ever return with her father, but I just feel like BB's disappearance, her eventual return and Mr. Arora's reappearance would create a serious impact.
Also, I absolutely detest recasts. A large amount of time needs to pass for me to warm up to a new actor playing a fan favorite's role. Mrunal has great chemistry with the cast. I'll be sad if we have to see her go and if the character is recast.