Story for Thriller Opening

Idea 1 - My first idea started off as the story of a young teenage girl, who has just moved into the house next door to lonely elderly man. She likes to spend her time with friends in public places; such as the park, a bus stop, up town, shopping but sometimes even her back garden. She is oblivious to the world outside of her friendship circle, and therefore does not realise her elderly, male neighbour stalking her wherever she goes.
The villain's background story was that he is an elderly man who, after having his high school sweetheart refuse his marriage proposal, has lived alone and hidden from the world, seeking a new partner.

After further research and reflection, I decided that this was too much of a cliché and would be to obvious as to who the villain is. Hence the second idea, which was used for the thriller.

Idea 2 - This time I decided that it would be best if the antagonist wasn't as obvious so that the audience would be kept guessing for a longer period of time, with the help of Roland Barthes narrative codes.
This time there would be a group of friends, between the ages of sixteen to nineteen, where one is 'obsessed' with another member of the group but keeps it a  secret. This obsessed 'friend' is jealous of the other friend (Melissa), so jealous and obsessed that the antagonist wants to become her but for that to happen  Melissa must disappear forever. The audience does not know this for definite until the end of the thriller. However in the thriller opening it will be obvious that someone is in danger as some objects with DNA on them, such as a hairbrush, will  start disappearing. This invites the viewer to assume the victim's horrible fate as she has been targeted and a shrine and plan has already been put into place.
Melissa will not know who to trust when weird things begin to happen but all her friends remain loyal and promise to help and protect her from the world. Little do they know that the future killer has already penetrated their barrier and is closer than ever.
The group of friends consist of three women and two men between the ages 16-19, in education or starting work to demonstrate the reality of the film as the audience can relate to the characters. It also makes it more thrilling as the reality of the film weights on the viewers as people that usually commit a murder, stalk someone, abuse someone and such are, the majority of the time, people we know and trust.