They should keep Lisa Trevor, I think. By adding Lisa Trevor to RE's history, you make Resident Evil's mytho's even grander.
Her story also gave depth to Wesker & Birkin's characters.
Instead of telling us Birkin/Wesker were cheesy, cliche evil villains, Lisa's history told us they were still reasonably Human, but were in the process of shedding layers of their Humanity. They were depraved and cruel in the ways in which they tortured/experimented on Lisa. Then theres the additional information on the G-virus, and how it had developed inside of Lisa over the course of the experiments.
L. Trevor is debatably one of the more frightening foes in REmake.