Reiver is just a archaic spelling of Reaver, nothing mysterious about it.Border Reivers - Wikipedia
The reason why I didn't include the Captain and Libby was that, at the end of the day, they're just upated models of those units, and don't really do anything interesting. They're good, s'all.
What got me excited about the reivers was the adjustments to the power armor that I don't think we've seen before. You'll notice the stripped-down look revealing fibre bundles, but also the narrowed legs and feet below the knee, which I tink adds a great deal of dynamism. The giant feet of regular marine always struck me as a bit... I dunno, weird. Why do feet need to be so big? What are they storing there?
EDIT: It's a bit funny that people find the skull masks silly when we've had Chaplains for decades.