Natalie actually has talent & has done a few respectable movies. While this will likely be her bigggest gig of all time, she has the ability to be one of the top 2-3 female boxoffice draws...as long as she can break free from the overexposure & typecasting. I don't think this should be too much of a problem. SHe has already released a couple movies after Ep.1. Ewan McGregor is also still doing "his" style of mivies after Ep.1. I guess the difference is that Lucas is using lesser-known, but very accomplished actors. Yeah, Harrrison Ford, went on to be great, but he was a complete nobody when he did American Graffitti & Star Wars.
BTW...Mark Hammill is a pretty highly in-demand voice actor, if I am not mistaken. Most notabley playing the voice of the Joker on the Batman cartoon series, but I believe that he stays pretty busy.