1. You do not have to worry about the possible cases what they can file is , if there was no discussion of dowry.
2. Legally speaking after the promise of marriage if you ddid have a sexual relation ship is chargeable under IPC.
3. Defaming can happen in two ways, legally and in the society spoiling the family name by spreading unwanted gossips.Legally they can not file anything unless you had taken money or gifts etc. and the marriage preparations have not yet stated to speak about an expense they have met,,,,,and also there was no sexual relationship or proximity or undue advantage from your part they can allege. So be cool do not worry about it. Good that you have decided to call it of before the marriage as you could avoid areal marital dispute.
4. If you apprehend that the family of the girl can file a police complaint , you can approach a session court anticipatory bail, but there is no chance police can simply charge without speaking to you or giving you a notice under the provisions of Cr.P.C to hear your part as per the recent 2014 ruling supreme court ruling. So Stay away from any sorts of contacts with the girl
consult an advocate locally and take the help in case of an emergency of any false police complaint by girl's family, if needed, you can send them an written intimation /notice through your lawyer stating the facts why you took a decision to call off the marriage.