Willow is described as the "big guns" for bringing Fred back; the witch was experienced both in dealing with souls and resurrection. Had there been a sixth season, Willow would have been brought in to separate Fred and Illyria.
Angel and Spike mentions cases of not staying dead, including Spike's return as a ghost after dying saving the world and Buffy's multiple resurrections.
Spike mentions hearing Willow was in South America, as Andrew revealed in Damage (2004) she was in Brazil.
In reference to Illyria's speed, Gunn lists Barry Allen (1956), Jay Garrick (1940), and Wally West (1986), comic book characters who all have been the superhero Flash.
Angel recalls having lost Cordelia after she was "violated" by some "thing", a reference to her possession by Jasmine that led her to a coma and ultimate death.