Mumbai, also known as Bombay, is the most populous city of India, and one of ten most populous cities in the world. It doesn't seem to have a flag — at least, I haven't found any.
So here's my proposal:
The seven stars represent the seven islands that constitute Mumbai. The building is the Gateway of India, the biggest tourist attraction of the city and representing its nickname "gateway of India"; it also appears in current coat of arms. The orange background was chosen due to its location in Maharashtra state, related to former Maratha Empire.
Comments are welcome.
Flags competitions are ont the way. I'll post my submissions here, too!