Unless otherwise specified, descriptions are moving left to right and top to bottom. All characters, unless otherwise labelled, were introduced in Kingdom Come (hereafter referred to as KC).
Page 1. Panel 1. The two characters seen here are Nightstar and Ibn al-Xuffasch.
Nightstar is the daughter of Nightwing (aka Robin, aka Kingdom Come's Red Robin) and Starfire, from the Teen Titans. The Kingdom Come Revelations supplement says this about her:
Ibn al Xu'ffasch is "heir to the empire of Ra's Al Ghul," who was one of The Batman's greatest enemies and another would-be world-conqueror.
The Revelations supplement says this about Ibn Al Xu'ffasch:
Originally, according to Waid and Ross, Ibn Al Xu'ffasch was tracked down and recruited by Ra Al Ghul's men to run Ra's empire after the latter's death. As we shall see, this piece of backstory has been negated.
Page 2. Panel 1. This is, as the text points out, Alan Scott's space station, originally seen in KC #1, page 33 (43 in the collected edition). The station is called, in the Waid/Ross Annotations to KC, New Oa, after the homeworld of the Guardians of the Universe. There are certain, seemingly-deliberate, visual similarities between the lower part of GL's space-city is the mothership from ELO's "Out of the Blue" album cover. The Revelations supplement had this to say about New Oa:
The title of the space station - "The Green" - may be, as Mark Coale points out, a reference to the elemental plane of nature introduced in Swamp Thing.
Page 3. Panel 4. Alan Scott's vulnerability to wood is from the Golden Age; just as the Silver Age Green Lantern, Hal Jordan, was vulnerable to the color yellow, so too was Alan Scott, the Golden Age Green Lantern, vulnerable to wood.
Page 4. Panel 1. This character is Manotaur, who was described in the KC card set as a "classical Greek myth armed for the future."
Page 6. Panel 2. The "Robin" seen here is Dick Grayson, aka Red Robin, formerly Nightwing, formerly Robin. The KC card set describes Red Robin as "formerly Nightwing, Dick Grayson, the original Robin, is following in his mentor's footsteps again."
Panel 3. The Orion mentioned by Red Robin is the New God and, in the future world of KC, the ruler of Apokolips (following the death of Darkseid at the hands of Orion).
Panel 4. Manotaur is "a good kid"? One more change from KC, where he was on the side of the bad guys.
Page 8. Panel 2. Tamaran is the planet, in the Vega system, from which Starfire comes.
"Uncle Vic" likely refers to the New Teen Titan Victor Stone, aka Robotman III, who in KC has become the liquid metal Cyborg.
Page 9. Panel 1. "X'Hal" is a sort of goddess of the species of the Vega system.
Panel 2. Nightwing's "circus parents" were killed by a mob boss - the incident which led to young Dick Grayson becoming the ward of Bruce Wayne and, later, Robin.
Panel 3. "Grampa Bruce" is, of course, Bruce Wayne.
Page 13. Panel 1. Tusk and N.I.L.8 are two characters from KC; Tusk is called by the KC card set an "elephant-shaped man-o-war." The KC Revelations supplement adds that Tusk is "...a Japanese giant robot armor design crossed with an elephant." N.I.L.8 is described in the KC card set as "a sentient armory with an appetite for destruction."
Page 14. Panel 1. Avia is the daughter of Big Barda and Mister Miracle. Mister Miracle and Big Barda are both DC heroes and Fourth World gods; Avia's parentage is reflected in her costume, which is a combination of the costumes of Scott Free (Mister Miracle) and Big Barda, and in her name - Avia was the wife of Izaya Highfather, the leader of the New Gods of New Genesis. The KC Revelations supplement says this about Avia:
Contrary to implications in KC, Starfire obviously didn't die, but left and returned to the Vega system.
R'yander is brother of Starfire.
Panel 3. Ollie Queen was, of course, the Green Arrow, and a major character in KC.
Page 16. Panel 3. "Mr. Secretary" is Secretary General Wyrmwood, from KC; he was Secretary General of the United Nations and gave the order for the nuclear bombs to be dropped, at the end of the story, on the rioting superhumans.
Page 17. Panel 2. The League of Assassins was an international secret society that hired its members out to kill for pay; they were originally part of Ra's al Ghul's criminal organization, but eventually split from them.
Panel 4. The Mankind Liberation Front, in KC, was the group of villains led by Lex Luthor.
This answers one of the minor mysteries of Kingdom Come: which side Ibn al Xuffasch was on.
Thanks to Mark Coale, as usual.