Right now, in order to 'clean my palate' before playing Mojika, I'm replaying Bloody Rondo, 3rdEye's first (and arguably worst) VN. A thought came to my mind that has been bothering me since I finished Oratorio Phantasm.
Are Shinkurou and Luna still alive at the time this VN occurs within the setting? The reason I ask is because
the true antagonist of the story is supposedly Shinkurou and Luna's great-grandaughter. Now, taking this fact into account, one has to ask... would Shinkurou and Luna allow something like that to happen to one of their descendants? She was stripped of her sanity and turned into a monster that could destroy two worlds by pure accident... but one has to ask why their son is the one intervening. I can't see Shinkurou ignoring that kind of thing, though I guess a century and a half of life might change him somewhat. Let's keep in mind that Shinkurou, by both being bitten, then drinking Luna's blood, stabilized as a vampire, so age and deterioration aren't the issues they would be with another artificial vampire. As such, it was made clear that he would live as long as he wasn't killed and Luna lived, so I honestly can't see him being dead at this point.
My hypothesis in this case is the simple fact that Luna, while she attaches firmly to Shinkurou, has a tendency toward melancholy and apathy that is marked throughout Bloody Rondo's length and seems to be endemic to all vampires who live beyond their first century. Hypothetically, I could see this happening to Shinkurou to an extent that he might be willing to stand back and let his descendants handle things themselves.
Now that we are finished establishing my hypothesis about why things in Oratorio Phantasm played out as they did, I thought I'd consider Shinigami no Testament.
Shinigami no Testament, Bloody Rondo, and Oratorio Phantasm share the same world (Sorcery Jokers didn't show a recognizable connection to the other games, so I'm viewing it as a completely independent setting). The events in Bloody Rondo apparently occur at least a few years before Shinigami no Testament, and at least a century and a half have passed since the events in Bloody Rondo when Oratorio Phantasm has occurred.
Shinigami no Testament has the weakest connection of the three, simply because if the inherently self-contained nature of its main storyline. To tell you why, [spoilers for those who haven't played it]
When the protagonist of Shinigami no Testament destroys the Black Manuscript, he resets all the events that occurred because of its existence.
The difference between Shinigami no Testament and Oratorio Phantasm is that Oratorio Phantasm actually has a direct link, whereas Shinigami no Testament just has a few pieces of info that tie it to Bloody Rondo.
Moving on... it is always interesting to see whether and how a writer or company will link its games together. Kinagusa Shougo's Akagoei and Reminiscence series are directly linked together in an obvious way:
Kizuna and Kaito from Akagoei 3 being present, Tae's descendant by Kaito being in control of the biggest tech company in Hope Town, and Yuki the robot being present in Hope Town.
However, it has never clearly been stated whether there is a canon for the two series (whether Kinagusa actually intended for the result that created Reminscence's setting to be absolute). Personally, I would prefer that it wasn't, because:
There are indications in Kaito's point of view that point to his presence being at least partly due to Reika's violent demise, indicating that they used part of her bad ending to bring about the current result.
Other games that possess a link are Ayakashibito and Bullet Butlers, through the Chrono Belt FD. In the Chrono Belt FD, Kuki-sensei is sent to the Bullet Butlers world along with a particularly nasty villain from Ayakashibito, and Alfred Arrowsmith is sent to the Ayakashibito world, where he, for the first time, confronts his own demons due to the essential peacefulness of the world he finds himself in (keep in mind that this is post-Ayakashibito's events). This link is a more peculiar one, in that I normally wouldn't have liked it... but Higashide Yuuichirou somehow made it work (seriously, I sometimes think Chrono Belt has more impact than the original games...).
Now, I just gave you a bunch of examples of games where the 'setting link' actually works out pretty well... but as anyone who has stumbled onto a 'bad sequel' knows, the 'setting link' is a sword that cuts both ways.
A more negative link would be the direct sequel to Hachimyoujin by Light... Bansenjin. Now, one of the problems with even thinking of making a sequel to Hachimyoujin is that the main characters had been stretched about as much as they could possibly be in the original. They had pretty much used up what made them interesting (which wasn't much in some cases), and Masada had pretty much played up their flaws and virtues as far as they could be... in other words, Bansenjin essentially revived a cast that had nothing new to add. There literally weren't any new angles within the existing cast that could be played on, and that resulted in a game that felt stale, perhaps precisely because Hachimyoujin was so self-contained. The new characters weren't that good in the first place, and Masada was really heavy-handed about how he screwed with the setting. As such, Bansenjin most definitely suffered from the 'sequel disease'.
It makes me wonder... why do some writers, regardless of their skill, seem to always want to make a bad sequel to an excellent game? Oh, Dies Irae far surpassed Paradise Lost, its predecessor in the trilogy of Shinza main-series games. However, that was more of a result of Masada peaking with Dies Irae than anything else.
Shimantogawa Seiryuu, 3rdEye's main writer, has obviously (seriously) grown since he wrote Bloody Rondo. Shinigami no Testament was immensely greater than Bloody Rondo, Oratorio Phantasm benefited from his realization that he wrote one-path stories better than multi-path ones, and Sorcery Jokers pretty much showed the peak of what he was capable of. Masada definitely grew after writing Paradise Lost and through the versions of Dies Irae (the first few of which sucked compared to the two final versions, Fabula and Amantes). However, he also peaked at that point, and the expectations created by the final versions of Dies Irae were impossible to fulfill... Higashide got out while the getting was good, recognizing that Tokyo Babel's financial failure meant bad things in the future (so quite naturally, he signed on with Type Moon). Shumon Yuu only ever seems to write when he has a masterpiece in mind...
This post was all over the place... but then, it was never intended to be coherent, since I was writing things as I thought of them. It is hard to make a VN sequel or reuse a VN setting... the adjustments necessary to keep expectations from ruining things for the reader are delicate, and few writers or companies can manage to do it well. 3rdEye did it by mostly keeping the setting links light, Masada failed with Bansenjin because he misjudged the quality of his own characters and setting, and Higashide managed to pull a masterpiece out of what should have been a massive failure...