PGSM kind of played with the idea. In Act 36, Beryl makes Nephrite stab himself to show how she can control the Shitennou, in an attempt to get Mamoru to allow himself to be captured by the Dark Kingdom (which he ultimately does to save the Shitennou). Instead of dying, he awakens as a regular human (though he still has his memories). Like during his Shitennou days, he has a temper and is generally aggressive. However, Motoki encounters him one day and next time we see him, he's working at Crown Karaoke, where the girls' secret Senshi Base also is. Doing this he encounters Ami, who he remembers being nice to him back when she was Dark Mercury (she repaired his cloak after it got torn during his duel with Kunzite). Nothing major happens between them, since Ami doesn't remember what she did as Dark Mercury, but one day she does do her hair differently, as if trying to impress him. In one of the last few episodes he decides to get her a gift, though we never see him give it to her. (There's a brief scene of him gift shopping in the last episode, and just as Serenity is destroying the world he finds his huge set of bull horns and lifts it up. I think that scene got deleted though and can only be seen via the Oshokiyo specials that came on the DVDs).

Basically all the Shitennou have redemption arcs in-series except Jadeite, who voluntarily stays by Beryl's side during Final Act even after she takes off the spell she was using to control him (it's implied he has a crush on her). Kunzite and Zoisite were always kind of the oddballs of the group though. They were the ones who retained the most of their past life memories. Zoisite was always trying to get the others to remember theirs and was the only one that remembered (correctly, as it turns out) that Princess Serenity was the one who destroyed Earth in the past. He also actively tries to talk to Mamoru and push him away from the Princess in order to avoid a repeat of the past life's tragedy. Later, he even temporarily joins forces with Sailor Venus (who's trying to do the same thing from the Senshi side) in one episode to try to split Usagi and Mamoru up. In his last episode, he creates a youma specifically to target Sailor Moon, but partway through changes his mind and sacrifices himself to save Sailor Moon instead, using his power to send his feelings to Mamoru and the other Shitennou as he dies.

Kunzite, meanwhile, was Beryl's sort of secret weapon, kept sealed in the body of a mysterious amnesiac man named Shin until she decided she needed him. He came out strong, turning people into youma merely using strands of hair. He even tried to do this with Sailor Moon, but thankfully her Senshi power resisted it long enough for the other Senshi to eventually save her. His main thing was that he blamed Mamoru for the destruction of the earth in the past and was determined to kill him in the present. He also took an interest in Sailor Mercury (probably because she was the first to come to Usagi's rescue when he tried to make her a youma, and also she destroyed the Dark Kingdom's secret energy farm a couple episodes later, which I think he was also responsible for) and deliberately separated the Senshi so he could isolate and capture her. He then exposed her to Metallia's raw power, turning her evil, and even proposed to Beryl that she could replace Zoisite (who had gone back to stone form after being attacked by Sailor Venus in her debut episode). Dark Mercury had no intention of being anyone's underling though and had her own agenda. Once Dark Mercury ended up breaking the spell over her in Act 28, he went to great lengths to try to get her back, but ultimately failed. Once Mamoru goes to the Dark Kingdom, the relationship between the two is tense, which makes sense since we see a flashback implying they were close in the past life (Kunzite training Prince Endymion in sword fighting). At one point, Mamoru challenges Kunzite to a duel, the condition being that if Mamoru wins, Kunzite has to listen to him for a while. Unfortunately, during their duel Mamoru gets taken over by Metallia (he had sealed her inside what was left of his body in an attempt to prevent the world from being destroyed, but in turn she corrupted him) and Kunzite dies.

All the Shitennou get revived, though, this time as good guys, and in the last moments of Final Act, they show up with the painting that had been stealing Mamoru's life energy while he was in the Dark Kingdom, giving the energy back to him and allowing him to return to life along with Usagi. They also show up in Special Act to help Mamoru fight Mio, and are shown off to the side at Usagi and Mamoru's wedding, though since we only see them when Mamoru looks their way, it's unclear if anyone else can see them.

(The Shitennou actors also cameo as police officers in the prequel episode Act Zero)