I would've tried to stay as true to the lore of the games as possible. For me though, I feel the main character should be Jill Valentine and noone else.
But first, I would've included a prologue sequence of around 5 minutes taking place in 1966 during Oswall E. Spencer's expedition to Africa with Ashford, Marcus and Bailey. After some quick encounters with snakes and crocodiles which kills some of their other team members, the prologue would then end with them finding the Stairway of the Sun, with a final shot of Spencer touching the flower with with a glorious smile on his face.
Then a montage along with the usual "Random Pictures presents", "A Film by Random Director" would follow, that shows images of the t-Virus being developed back in the states, along with newspaper clips of Umbrella becoming the world's biggest pharmaceutical company, and so on. The montage would end with a "30 years later" text.
After that, the movie would start and pretty much follow the story of the original RE, with some alterations and additions to fit the movie format. For example, I would've first included a scene taking place at the Raccoon City Police Department, where Chief Irons holds a special meeting for Alpha team together with Captain Wesker. This would take place in broad daylight, showing Raccoon City as being vibrant and full of life. This special meeting will be important to establish our characters and their relations, and how close friends they are (especially Jill and Chris) and how they like to joke around with each other. Chief Irons will explain that Bravo team has gone missing up in the Arklay Mountains, and that this information hasn't been revealed to Alpha team until now. It's important to show that both Irons and Wesker DON'T want to rush and investigate their loss of contact right away, and that they have to give it some time before sending a rescue mission since they don't yet know what has happened. But most members of Alpha Team will be reluctant to wait, and will insist on preparing for a mission that same night. I also think it would be interesting to portray Wesker as a non-villain from the start whom is unaware of his origins, instead showing him as being kind and assertive at the same time. Its much more likely that this version of Wesker would be respected by his team members.
Everything will then proceed as normal up until they reach the Spencer Mansion. To make for a more interesting film, I'd place Jill, Chris, Barry and Wesker all together in this scene, before they decide to split up 2 and 2. Wesker will begin to feel strange and have seizures, and say that it feels like the mansion is speaking to him. One way or another, both team-ups will run into trouble and split up as well, and then Jill will be found by Chris. I'd cut out Rebecca from most of the plot, instead focusing on Jill and Chris's friendship, as well a completely new plot involving Wesker finding a source in the mansion that reveals his origins - turning him insane.
There would of course be some focus on the mystery aspect of the game as well, something which I think is vital to the original story. Jill and Chris will find clues leading up the devastating story behind Umbrella and the creation of the virus.
Of course, there will be action too, but I'd limit the action to being more of self defense during scary attacks with zombies suddenly popping up. Then, I would have the Tyrant being released earlier around the film's midpoint, since I think a Terminator-like hunt after Jill and the others through the mansion would make for some very scary and exciting moments. I think team members such as Richard can die during this hunt, to show how dangerous and brutal the Tyrant is. Insane Wesker will of course be an interesting aspect of this part as well, and in one scene near the end he will submit himself to the Tyrant on the grounds of the mansion and be seemingly "killed" by it. Of course, the film's climax will show Jill and Chris with the help of Barry and Rebecca finally defeating the Tyrant and then escaping the mansion in a helicopter, before blowing it up.
The final scene of the film will show an Umbrella helicopter landing near the destroyed mansion, with two soldiers picking up the wounded Wesker and carrying him onto the helicopter where none other than Oswall E. Spencer sits. Once Spencer is told by one soldier that Wesker seems to be alive, Spencer simply replies "Good. We have much work to do" and the movie ends.
That's my take on it. Of course there are many people who would strongly dislike what I did to Wesker's character, but I think this way works better in a film adaptation. Also, I think by telling the story like this it sets up sequels in a much better way.
