Now i could be wrong and there could be information already out that rules out what im about to say but I'll just write this up.
Okay so I think you will start off as Chris first and you are in china because he has been informed of a bio terrorist group in the country. So moving on as you see in the trailer Chris is the one shooting other people (the terrorist) and I think that somehow a virus of some sort slipped by Chris and his team and was sent to tall oaks. Chris then finds out about this and heads to tall oaks. ( possibly ada had the virus)
Now switch to Leons part. Leon is with the president talking about the raccoon city incident when the outbreak happens. So the president turns in a zombie and Leon is forced to shoot him. So then Leon gets a call from Hunnigan (sp)? and is told about the outbreak in the city and that most if not the whole town has been infected.
**Note** Maybe Helena unknowingly gave a virus to Ada who was at the time working for the government and somehow got her hands on the virus. Obviously completely out of the way of Leon. Knowing how Ada is she is known to betray anyone no matter who they are
And that's all I got for now I'm not sure sure how the Mystery man ans imo Ashley fit into this --- Ramon Salazar: I've sent my right hand to dispose of you. Leon S. Kennedy: Your right hand comes off?