Expanded mutation mechanic?

[MacMillion]

How difficult would it be to create an expansion to the mutation mechanic? As it stands now, mutation of legendary enemies is really lame. They just start emanating a colored aura and get all their health back. I propose that instead of this, they regain their health rapidly rather than instantaneously and start to undergo a hideous transformation/growth much like Resident Evil or The Thing. They'll gain new attacks and start behaving/moving erratically. I believe the effect would be terrifying and awesome at the same time and make fighting legendary enemies really fun, interesting, and perhaps a bit more challenging.