A Mortal Kombat 1 leaker called bac736, who revealed Darrius, Sektor, and Cyrax before their announcement, Reptile having a human and lizard form, and Smoke's design, claims that Shujinko is "one of the last Kameos" to be revealed. They sent the Mortal Kombat Leaks subreddit mods pictures to prove the claim, and after viewing the evidence, the mods say "it seems as legit as it can get".
Shujinko first appeared in Deception as an adventurer with dreams of fighting Shang Tsung, a villain who is also present in Mortal Kombat 1. In his debut, he was a young teenage boy, but his most iconic look is from when he's older with a long grey beard and hair to match. He was last seen in Mortal Kombat X as a cameo in Cassie Cage's ladder ending, with a brief mention in MK11. However, he hasn't been playable since Armageddon.
"I haven't seen much actual confirmation from other people but Shujinko is one of the last Kameos," bac736 said. "In the image I have it's him and then the last slot is a silhouette with a question mark on him so I can't tell if it's a random select or a secret Kameo." As for why bac736 hasn't released visual evidence to the community, they say it's because their "source could get in serious trouble".
Shujinko has an interesting story both in the universe of Mortal Kombat and the real world. The series was in a rocky place after the divisive reviews of MK4 and the less-than-desirable spin-offs. Cue Deception, which revealed that the Earthrealm warriors had died failing to conquer the Deadly Alliance. Without their protection, Dragon King Onaga was ready to claw for power, which meant a new hero needed to step up--Liu Kang was dead and Raiden was corrupted. So, Midway introduced a new character, Shujinko, which is literally Japanese for protagonist.
We're introduced to him standing next to Quan Chi and Shang Tsung, hardly the most heroic of beginnings, while he claims to have revived Dragon King Onaga, the next big bad. However, he also claims that only he can destroy Onaga. But after all this hype and Ed Boon going as far as to declare him "the next generation Liu Kang", you cant play as him outside of Konquest Mode, an open-world RPG akin to Street Fighter 6's World Tour.
He wasn't the most popular character, but he became a cult favourite over the years, much as many of the old 3D-era Mortal Kombat games themselves did. So, maybe MK1 will be his time to shine after so long, reintroducing him to a whole new generation.