While no character from Moonlight Syndrome shows up in the 25th ward, the last chronological chapter in the game, Yuki, takes place in Hinashiro.
The whole set up of Yuki, is meant to serve as a spiritual sequel to the Syndrome games. Yuki Shimohira, a girl who is able to sense spirits or ghosts, is set up as inheriting the role that Yukari Hasegawa, Chisato Itsushima and Mika Kishi had before her, that of a supernatural investigator.
While some fans seemingly took this as a tease for a new upcoming Syndrome game. I know for a fact the rights of the Syndrome franchise, are not in the hands of GHM at all. (The Syndrome franchise is owned by Spike-Chunsoft, & the original source code for the games has been lost to time.)
While a collaboration is not off the charts, I believe Yuki merely acts as a thematic coda to the Silver Case setting, bringing things full circle back to the start of the storyline.
The Silver Case:
The connections between TSC and 25W are surprisingly linear, so I won’t spend too much time dwelling on it.
The 25th Ward takes place in 2005, in the eponymous 25th ward, a new urban area meant to further refine the concept of “perfect society” that the 24th Ward didn’t manage to achieve.
Kinshiro Nakane is apparently in charge of the 25th ward’s development. One might infer from his surname that he is related to Ginji Nakane, the chairman of the TRO/CCO alliance in 1979, and that he presumably inherited a measure of power through his bloodline.
The game does not hide its status as a sequel, contrary to Flower, Sun and Rain, therefore references to the Maspros and the Shelter Children Policy are abundant and explicit.
Returning characters include Sakura Natsume, Sumio Kodai, Tetsugoro Kusabi (in a flashback), and most importantly Michiru Kosaka, a minor character in TSC promoted to a pretty major player in 25W.
The player character of 25W, a nameless character who is eventually christened as Kamui Uehara by Kinshiro Nakane, might or might not be the same main character as TSC. I tend to believe he isn’t, because Sakura does note that the two look alike, but does not recognize him as being the same person despite having worked with him in the past.
Tetsugoro Kusabi is not present in the game outside of a flashback sequence, because he is conducting an investigation in the United States.
Flower, Sun and Rain:
During the course of the game, RAB agent Osato makes use of a miniaturized version of Catherine, the code breaking machine used by Sumio Mondo in Flower, Sun and Rain.
Sumio Mondo returns in the game as well, though under his original persona of Sumio Kodai, and seems to have retained his abilities as a searcher.
Meru could have been a victim of the Euro Maspro Shelter, though the game does not confirm this outright.
(She mentions being part of the third batch of Shelter Children, which might be referring to the Lospass Shelter, but the details are unclear; she may have been referring to an otherwise unknown batch of kidnappings.)
The system engineer behind the 25th Ward may or may not be Stephan Charbonie from hotel Flower, Sun and Rain; though he is never shown, he does have a similar speech pattern and mentions his inspiration.
Mokutaro Shiroyabu/Shimohira might be part of the Next Maspro mentioned at the very end of Flower, Sun and Rain. His multiple, identical bodies are definetly reminiscent of the cloning/replicant technology seen in Lospass Island’s Shelter at the very least.
The idea that each Shiroyabu was living its own life unaware of the others, helps to retroactively explain the appearence of sixteen Mondos at the end of FSR; It can be inferred that before being summoned to Lospass Island, each Sumio might have been leading an independent life, though this is never actually spelled out within the game.
No More Heroes:
The greatest point of connection between the No More Heroes series and The Silver Case games happens in Travis Strikes Again.
One of the endings of The 25th Ward sees Kamui Uehara being transported into the future by Red and Blue to assist a certain Mr. TD.
Uehara actually does show up in Travis Strikes Again, and this scenario is played out in the first and sixth chapters of the Visual Novel, Travis Strikes Back, revealing Mr. TD as being Travis TouchDown himself:
Not only that, but Travis actually travels to the 25th ward itself (renamed to 0 Ward after the controlled demolition seen at the end of the game, reframed as an artificially induced earthquake called the “Rage Earth” in TSA) in the fourth chapter, “Modern Observation”:
Returning characters include Chiruko/Chillko Takahashi (I suspect Chiruko, the ESPer girl seen working with Koshimizu Yoshimitsu, and the bike delivery girl seen working with Shinko Kuroyanagi, might have been initially intended to be two different characters, seeing their different portraits, but in the remake they seem to have been fused into a singular character, and that’s how they appear in TSA), Mokutaro Shiroyabu and Kousuke Kurumizawa, who has relocated to the building where Kinijo’s machiko was living during the events of The 25th Ward.
(Kinijo, aka GoodLookingGuy, was in league with Kurumizawa during the events of 25W; making it possible that Kurumizawa was the original owner of the building.)