This is more of a story idea, then a theory I think will happen, but.
Spencer used the same virus variant Wesker used to fake his death in RE 1. With Wesker dead, Spencer goes to the public claiming Wesker to be the real bad guy at Umbrella, and proposes he brings back Umbrella to continue medical research(not really). Being that Wesker nearly destroyed the world recently, everyone believes spencer and allows Umbrellas restart.
Spencer then uses the Projenitor Virus with some of Weskers DNA, to create A clone Army
P stay with me)of weskers, then gives them all a new strand of Virus to enhance their abilities without mutating them, one that reverses those same abilities on anyone else, creating zombies. It also allows for minor mind control(like how other BoW's could carry out certain missions).Where did he get this strain? Well it's been dormant in little Sherry Birkin ever since her dad put there when she was first born. somewhere no one would find it. Since Wesker is no longer part of the Agency they release her and she is immediately captured by spencer.
Claire finally comes back for Sherry, becoming our new protagonist, along with a close friend of her brothers, Barry Burton.
Where is all this taking place? In a recreation of the original Spencer Mansion of course, complete with all new traps/puzzles, as well as a rebuilt raccoon city, as Spencer has purchased the land, and intends on rebuilding it into a metropolis, as raccoon was the home of Umbrella afterall.
My idea for the final boss, instead of using the standard main antagonist injecting themselves with the virus, is that while building the new spencer mansion, the workers are attacked by the one and only Lisa Trevor, who Spencer traps, and improsons, and so to not completely depart from the newer games, he injects with las plagas and uroboros. Given her resiliency to most virus's, these don't completely transform her, but give her new abilitles and slightly, different aesthetics(i dont want to fight another black leech looking thing in the shape of a human).