IMO a trilogy should be RE1, RE2 (possibly with some bits of 3 thrown in, like Jill joining Claire and Leon's motley crew of survivors and Nemesis chasing her), and then a loose adaptation of Code Veronica.
I say a loose adaptation of Code Veronica, because I like the concept of CV for the most part, but I think that it kind of got stupid in the third act and it does not offer a thorough enough resolution for the end of a movie trilogy. I like the idea of a zombie movie set in a prison, and they toyed with the idea a little in RE Afterlife but they didn't really explore the possibilities.
Rather than just being a private prison where Umbrella ships their enemies, I would make it so that is actually a "legitimate" island prison similar to Alcatraz that houses real criminals. Prisons in the US have been privatized, so it is not that far fetched that one of Umbrella's subsidiaries would own and operate a prison that would also have secret levels to house secret prisoners and to conduct experiments on the inmates. The wealthy Umbrella brat Alfred Ashford would be the prison's warden as in the game, and he'd have an implied squicky relationship with his sister who has been missing for quite some time. However, there would not be a part where they for some inexplicable reason go to the South Pole. Pretty much the whole thing would take place on the prison island, and the movie's main focus would be on the "zombie outbreak in a prison" concept.
Claire and Steve Burnside would probably end up banding together with some of the other inmates to fight off the infected, and later some of Umbrella's monsters escape (or are released? *dun dun dun*) from their underground pens and start hunting the prisoners too. It could be a very gritty and intense movie if it were done correctly, and it could serve as an interesting social commentary about the US prison system.