I would prefer more voice actors, rather than celebrity voice actors. The second I heard a beggar talk to me with the voice of a Imperial noble, and then say goodbye to me with the voice of a Nord, I knew I was in for a bumpy ride.
Or it could be done how New Vegas did it, with a bunch of semi-celebrity voice actors mixed in with a bunch of regulars. They had like 15 or so Hollywood actors, but no real big names. I followed Mr. House in the main quest simply because I wanted to hear Rene Auberjonois (Odo from ST: Deep Space Nine) nail all of his lines. Micheal Dorn (Worf from ST: TNG and ST:DSN) returning as Marcus, Matthew Perry (Chandler from Friends), Kris Kristofferson, Micheal Hogan (Col. Tigh from Battlestar Galactica), John Doman, Wayne Newton (famous Las Vegas Singer), William Sadler (Shawshank Redemption, Green Mile)..... I could go on.
I loved New Vegas's dialogue, but my favorite dialogue sequences were from Chief Hanlon, Marcus, Doc Mitchell, Benny, Victor, and most importantly, Mr. House. It's no coincidence that it was the Hollywood voice actors. You don't have to go out and get Patrick Stewart and Sean Bean, as amazing as they are. Plenty of mid-level guys that can do the job too, and you can afford more of them.