You could have a young Sean Connery back as James Bond.
Sigourney Weaver appears to drop 20 years whenever she slips her consciousness into an alien body in Avatar. But Cameron’s facial scanning process is so precise—zeroing in to the very pores of an actor’s skin—that virtually any manipulation is possible. You may not be able to totally replace an actor.
However, you might replace or enhance an action actor with a better actor and then use the CGI to change the appearance. Arnold Schwarzenegger could then have a ghost-actor be scanned who might have a greater emotional range. Say someone with the acting range of Robert DiNiro or Robert Downey Junior could have provide the expressions and acting for an action star.
Dead actors and artists could be revived or actors living now could continue their franchises after they are gone.
There is also the increased possibility of fake news interviews. The image of President Obama could be made to say or do anything. Similarly for Osama bin Laden.