I really liked Aggregor and the Highbreed. Aggregor was a pretty serious threat, and instead of having henchmen or minions, he preferred to handle things himself.
I liked how the Highbreed were an entire race instead of one character, which IMO was a cool way to raise the stakes at the start of AF.
And if I may on Khyber, I believe what Tyranidomega meant was because of Khyber's limited appearance he did not live up to be a serious threat that Tactical Ochoa thinks he could've been.
Have to ask Tactical, how would you have Khyber grow to become a serious threat? I have some ideas myself, like maybe he resorts to kidnapping one of Ben's family.