Update

Because of the research we did into the suspense built in thrillers, we felt confident doing it ourselves and wanted some suspense in our opening. This is why we built on our initial idea of someone cleaning blood in a bathroom. The new plot is that someone knocks on the door and the character cleaning is scared; this would suggest to the audience that it's the police or someone bad at the door and therefore create tension. She opens the door at the climax of suspense and it's just a sales person. We did a quick draft, it's not the complete plot but here it is:  

We used hand held shots from behind and high angle shots to make the character look vulnerable. The close-up of her face lets the audience know she is scared, in turn making them scared. The shot of someone at the door is good because we can't see her full identity which builds suspense. At the end we had a little play around with the audio, character cleaning up seems to be in a dream like state; she does't know what's going on because of the severity of what she's done. We think we should and could do an opening with less of a plot than this and still get across the genre through music and lighting and angles. So we've concluded that we'll just have her cleaning up the bloody bathroom. This draft has allowed us to build on our skills meaning we'll have even more confidence during the process of our final opening scene.