I'd rather see a new face play Magnum than a well known actor, only if the do a prequel story. But I really hope they do a movie with Selleck, Manetti, Mosley, and Hillerman. Have the daughter of Magnum play a big part in the story, and who knows maybe T.M. had a son in the 90's as well.
Without Tom Selleck, this is going to be heading the way of that infamous Miami Vice movie. Neglected by or - even worse - disappointing interested loyal followers, basically unable to be a movie in its own right (it it were that it would not need to draw on the famous show), doomed.