I would like it to mostly be unknown actors, which worked well with the other films. However, I think that it should have Sam Neill (because I've loved him ever since I saw Jurassic Park for the first time, and he's about due for another film; the last thing he's been in was a CBC miniseries) and Sharlto Copley (from District 9). Both men are great actors, and I love the South African accent that Copley has.
P.S. Someone suggested Vin Diesel?
Vin Diesel? AAAAAAAAAAGGGGGGGGHHHHHHHHHH!!-
BOOM!