Richard E. Grant should have been Donald Pierce...
... Boyd Holbrook should have been Pretty Boy, and the Reavers should have included Bonebreaker and Skullbuster.
They should have just doubled down on the Reavers and made them more cyborg-y. It would have made the villains a little more distinctive and satisfying ... as it was, they were just a nondescript-looking gang of ineffective grunts.
Zander Rice was ... meh. The revelation that his father was a guard killed during Wolverine's escape from Weapon X had no impact because the character was so under-developed.