DC Comics' 1990s Kingdom Come series was not only influenced by DC's Golden Age history, but the series has been quite influential on subsequent DC projects. I'll first mention some of Kingdom Come's influence on the mainline DC universe and then I'll mention some of the Golden Age concepts that inspired parts of the series.
Kingdom Come's Influence:
Earth 22 - In the new and rebooted multiverse that followed events of 52, it was revealed that the Kingdom Come story actually took place on Earth-22. Of course now that the entire multiverse has been rebooted with the new 52, who knows?
Red Robin - The first character to don the Red Robin costume was Dick Grayson, formerly Robin, of the Kingdom Come universe. In this reality he was married to Starfire (now deceased) and their grown daughter is Nightstar. The Red Robin mantle in the mainline DC universe has never belonged to Dick Grayson, but the first Red Robin was Jason Todd in Countdown to Final Crisis # 17 where Todd dons the costume as he fights alongside the Batman of Earth 51. The current Red Robin of the new 52 universe is Tim Drake, another former Robin.
Ibn al Xu'fffash - In Arabic this means "son of the Bat." It was revealed in Kingdom Come that Bruce Wayne and Talia al Ghul had a son. In the mainline DC universe he is the latest Robin, the boy wonder Damian Wayne. In Kingdom Come there is a romantic spark between Bruce Wayne's son and Dick Grayson's daughter. In Justice Society of America, volume 3, # 22, they are shown twenty years later with two children attending the Earth-22 Batman's funeral.
Red Arrow - Roy Harper, the former Speedy, all grown up. He made his first appearance as Red Arrow in Kingdom Come # 2.
Maxine Hunkel - The altar ego of Earth-22's Red Tornado. Maxine's New Earth counterpart is the granddaughter of Mathilda Hunkel, the original Red Tornado. Maxine first appears in Justice Society of America, volume 3, # 1 and she adopts the name Cyclone in the third issue. It remains to be seen, as far as I know, what and where her place will be in the new 52 multiverse.
Atom Smasher - The godson of the original Atom, Al Pratt. Atom Smasher was known as Nuklon in the first Infinity Inc. series.
Wildcat - In Kingdom Come he is a man-panther possessing the spirit of Ted Grant, the original Wildcat. On New Earth the man-panther version turns out to be Grant's long lost son. He at first shares the name Wildcat with his father, joining the JSA in Justice Society of America, volume 3, # 4. Upon switching to the JSA All-Stars team, the man-panther changes his code name to Tomcat. It remains to be seen where his place will be in the New 52 multiverse.
Lightning - Jennifer Pierce, the metahuman daughter of Black Lightning, first appeared in Kingdom Come. Her New Earth counterpart was first introduced in Justice Society of America, volume 3, # 12.
Blue Beetle - The Kingdom Come Blue Beetle wore an armored battle suit which I suspect might have served as inspiration for the Jaime Reyes version of the character in the mainline DC Universe.
Influences Upon Kingdom Come:
The Flash - The Flash of Earth-22 wears a helmet similar to that of Jay Garrick, the original Flash.
Red Tornado - The oringal Red Tornado of Earth-22 is Mathilda Hunkel, active and armed to the hilt in Kingdom Come.
Bulletman / Bulletgirl - Formerly of Fawcett Publications, this duo was very popular in the 1940s. The Fawcett heroes were purchased by DC Comics in 1972. Of the notable appearances of this duo are Justice League of America # 136-138 and the Power of Shazam series in the 90s.
Living Doll - Appearing on Superman's shoulder in Kingdom Come #3, she is the daughter of Doll Man and Doll Girl, a Golden Age duo purchased by DC from Quality Comics.
Phantom Lady - Another Golden Age character purchased from Quality, she appears in Kingdom Come as a literal phantom resembling Bettie Page.
Huntress - An African version of the Golden Age character, Paula Brooks.
Vigilante - Based on the Golden Age motorcyle cowboy, the Kingdom Come version is part robotic with a Gatlin gun for one of his arms.
So there we have my rundown of charaters from Kingdom Come that I find particularly intriguing due either to their Golden Age inspiration, or to how they have subsequently inspired groups like the Justice Society and the Batman Family whose origins date back to the Golden Age.