the other day, I recently re-read Kingdom Come via a copy of Absolute Kingdom Come, paying the most attention to the copious amounts of supplementary material in the back. While reading through all the names on the two page section marked "Memorial," which was basically a key to the 105 (105!) characters on the covers of the first three issues, I was actually taken aback by how many of these characters were introduced into the mainstream DCU before the 2011 reboot. Several of them were introduced by Ross himself during his collaboration with Geoff Johns on the 2007-launched Justice Society of America (as character creator, cover artist and eventually co-writer), but there were obviously a lot of Kingdom Come fans writing and editing at DC Comics in the last 15 years.
Just looking at the key...
•A female Judomaster II was introduced in 2007's Birds of Prey #100, and she later joined the JSoA
•Magog was introduced to the DCU in the pages of JSoA, eventually earning his own short-lived solo title, and even making the transition into the post-Flashpoint, rebooted New 52 universe in the pages of Superman/Wonder Woman
•Lightning, "Black Lightning's metahuman daughter," was introduced in the pages of JSoA, albeit via a retcon that gave Jefferson Pierce his second adult daughter (Writer Judd Winick had previously retconnd Black Lightning's history to give him an adult daughter who took the name "Thunder"; Black Lightning's retconned origin would include them in Black Lightning: Year One
•Von Bach, Swastika and 666 are among "The Fourth Reich" super-Nazis that mess up a picnic in Columbus, Ohio at the opening of the 2007 Justice Society of America series
•The Thunder of Kingom Come is "a new Johnny Thunder with the mischeivous spirit of the Thunderbolt;" the Jakeem Thunder that Grant Morrison, Howard Porter and John Dell introduced in 1999 JLA arc "Crisis Times Five" has some obvious differences from this Thunder (including having an exterior rather than interior Thunderbolt), but they sure look an awful lot alike
•The Hawkman of Kingdom Come didn't come to the DCU, but his design did, as former Infinity, Inc. character Northwind "evolved" into that design during Johns and company's run on JSA
•Robotman III is the name that Victor Stone, AKA Cyborg takes in Kingdom Come, sometime after his half-robot body was updated to one of golden-colored liquid metal. Cyborg never took that name in the DCU, the second Robotman never stopped using it or died for very long, but he did acquire a golden-colored liquid metal body at the climax of Titans/JLA and kept it through Devin Grayson and company's The Titans ongoing. Johns changed Cy back to a more familiar design in the pages of The Flash, essentially giving Cyborg his George Perez-design, but with the gray metal parts now being colored gold. When Johns relaunched Teen Titans in 2003, Cyborg was back to the grayish metal coloration
•While I've lost track of DC's Starmen over the years, Kingdom Come's Starman VIII–his costume and origin as revealed on this key completely in tact–was a member of the JSoA
•Wesley Dodds's former sidekick Sandy the Golden Boy did indeed dawn the costume of Kingdom Come's Sandman IV (that of Jack Kirby's 1970s Sandman), in the pages of JSA. He didn't keep it long, though, and never really took the name Sandman for very long either, sticking with the one-syllable "Sand" for most of his career
•The red-haired, tornado-powered Red Tornado III was introduced with a different name and cooler costume into the pages of JSoA as Cyclone; she's sadly been MIA since the 2011 reboot
•Atom-Smasher was probably the very first emigree from Kingdom Come to the DCU, and he also stuck around the longest. Here he's described simply as "formerly Nuklon, godson of the original Atom." Nuklon took the name Atom-Smasher in the first story arc of James Robinson, David S. Goyer, Stephen Sadowski and Michael Bair's JSA, although his costume varied somewhat (and for the better). He wore the blue full-head mask that Ross' Atom-Smasher does, but had a more traditional spandex superhero costume than the professional wrestler gear Ross outfitted him with. Atom-Smasher, like most legacy heroes, has also been MIA since the reboot
•The Red Robin name and costume came to the DCU via Jason Todd during the best-ignored Countdown period of DC Comics, and was then adopted by Robin III Tim Drake, after it became clear that Robin IV wasn't going to be killed off any time soon. After the Flashpoint reboot, Tim Drake ditched the Kingdome Come costume, but kept the name. The name has apparently become strong enough that I noticed it was what was used instead of "Robin" in this goofy Batman Unlimited: Animal Instinct direct-to-DVD cartoon I watched (the costume in that looked like a stripped-down, anime-inspired version of Ross' design
•Roy Harper, who first went by Speedy and then by Arsenal, never grew out his goatee or started wearing a red cap, but he began wearing a red version of the Neal Adams-designed Green Arrow costume in Dan Jurgens' Teen Titans way back in 1998. His costume changed (for the worse) a few times since then, but he returned to a red Green Arrow costume in the pages of the Brad Meltzer/Ed Benes Justice League of America, where he also finally took the name Red Arrow. He later returned to "Arsenal" and a different costume, and, since the reboot, has been going by Arsenal, but wearing a costume closer to that of his Red Arrow one
•Zatara II was introduced to the DCU during Johns' Teen Titans run, but rather than being the "son of the late Zatanna, and grandson of the original Zatara," he was the still-alive Zatanna's cosuin.
•Wildcat III is described as "a man-panther with the original's spirit;" a Wildcat with that exact design was introduced by Ross and Johns into their JSoA, although he was now the long-lost son of the original Wildcat, Ted Grant
Those are just the characters from the covers of Kingdom Comes #1-#3, of course, and thus not a complete accounting.
It's also interesting to note that in the years since these covers first appeared on comics shelves other characters with these names have all appeared, but in very, very different forms. Characters like Mr. Terriffic II (all-around good guy Michael Holt, rather than the over-equipped and over-zealous human arsenal of Kingdom Come), Joker's Duaghter II (a crazy lady with a belly shirt and The Joker's flayed-off face worn as a mask over her own, rather than a Jill Thompson-esque Harlequin with cool gadgets), Hourman III (an intelligent machine colony from the 853rd century with vast powers over time and space, rather than a strong guy who can fly), Batwoman II (a mundane human vigilante, rather than a New God fan of Batman's with an amazingly insane costume and a giant flying dog named Ace), Spy Smasher II (a lady who messes with Oracle in Birds of Prey, rather than an "indpendent operative in the post-cold war world"; neither is terribly developed, but their designs are racially different), a new version of Black Condor and The Phantom Lady (Basis updates of the originals; Kingdom Come's Condor wasn't really distinguishable from any previous Black Condors, but its Phantom Lady was an actual phantom) and a new Stars and Stripes (Plucky teenager Courtney Whitmore and her stepfather Stripey in a robot battle-suit code-named S.T.R.I.P.E., rather than the heavily armed, reckless legacy versions of Kingdom Come).