Disgaea 4: A Promise Unforgotten/WMG

Volcano is Vulcanus' Daughter and nothing more.

 * Even if it would be, I'm having a hard time imagining this guy being reincarnated as This. They'll probably hint at this for a while, though, just to throw players off track. My guess is that Vulcanus will think that Volcano (regardless of what she's actually doing) is betraying him, and sets up a complicated but ultimately kinda stupid Xanatos Roulette to try and get her back, while the real villan (whoever he is) actually poses a threat to the party. It would be even more awsome if Vulcanus somehow teamed up with  and  (the villans from the other two games), but that's more wishful thinking than anything.
 * Honestly debunked. Volcano's true name was revealed in a late-game scene.

Flonne is a candidate for Vulcanus's old job.
She's studying to be an Angel again; and we can see her Angel sprites in the promo art; so it's going to be part of the main plot. Flonne can at least pretend to be an Archangel as seen in Raspberyl Mode in Disgaea 3: Absence of Justice. Volcano is the other candidate.
 * Jossed. She IS the Archangel and Volcano's superior.

Disgaea 4 will go Meta-recursive, and will make its serious moments an Over the Top parody of its own Sliding Scale of Seriousness Versus Silliness
The second Promo suggests it. And the games into video has hints of it too.
 * Confirmed. One scene has the characters being depressed over succumbing to the recently spreading "A Virus".

The obligatory second mode will star Fuuka
There's already at least one piece of art with her in the human world - in her "pre-Prinny" life. The second game mode, like the Axel mode, will be a prequel to the main story and cover Fuuka's descent into delinquency, but uniquely, will take place in an alternate modern day human world, filled with monsters and the like.
 * NI come out and said that they are indeed planning a DLC mode for a popular Disgaea 4: A Promise Unforgotten character and Fuka is one of the more popular characters of Disgaea 4 so...
 * Confirmed. The secondary mode will be about Fuuka and Desco.
 * I have come up with a possible plot for the scenario which will star both Fuka and Desco if the recent Disgaea 4 popularity poll is any indication:
 * They have already taken over the world and now Desco is acting out her role as the final boss. Unfortunately, every hero who comes to their doorstep falls easily and doesn't give Desco that thrilling climatic final battle feel. She gets more and more depressed as one by one, the heroes fall to her. In the final chapter, Fuka betrays Desco, which really pushes her off the deep end and the two engage in one final heated battle. Desco finally falls, but Fuka tells her that she betrayed her only because she didn't want to see her depressed anymore and wanted to give Desco the final battle she had been craving for a long. Desco is so overjoyed to see her Onee-Sama care so much about her, but Fuka tries to brush it off Tsundere-style.
 * Alternatively, a Nonstandard Game Over could be added to the final battle in which Fuka loses and Fuka, with her dying breath (At least if she's still capable of actually dying at this point), tells Desco the true reason for her betrayal and this is when everything goes to hell, possibly to make up for the lack of a Downer Ending in the main game.

Desco and Asagi will team up.
Asagi wants to star in her own game; Desco wants to become a final boss. Clearly a match made in the Netherworld, and they simply must join forces—all with the intent of eventually facing off in their very own spotlight title, of course...!

The Netherworld the game takes place in? Axel's home world!
Disgaea 2: Cursed Memories: Dark Hero Days has the last chapter taken place in a children's orphanage where building contractors have been commissioned to build an Overlord's castle. Axel stops them, of course, but that doesn't mean the castle doesn't ever get built. Cue Time Skip, and Overlord's castle is finished, with the antagonist as the Overlord. That explains how the Dark Hero is without a cameraman or director in sight; he's home!

Valvatorez has a direct connection with this Netherworld's previous Overlord.
In all three of the previous games, the main character had a direct connection with the previous overlord (for the first and third ones it was the main characters father and in the second it's ) So going by this pattern, it seems almost obvious.
 * Although Valvatorez and Netherworld President Hugo are eventually revealed to have been rivals long ago, their rivalry is not a prevalent part of the story, at least compared to the previous games.

Valvatorez will be voiced by Yuri Lowenthal.
He looks the part...
 * Most people are saying Johnny Yong Bosch due to the similarities to Lelouch.
 * Jossed. It's Troy Baker.

The names of angels are actually Nom De Guerre.
It would explain why there are now two people named Vulcanus: the events of the first game led to the first becoming Unpersoned, and his Code Name and ranking were assigned to someone else. As for why Flonne got to keep her name after becoming a Fallen Angel, she never exactly disobeyed Lamington's orders...plus, the audience is too used to calling her Flonne by now.

Fuka's father wanted and needed Fuka to die.
Given his reaction to seeing his dead daughter, either he truly is insane, or having her die was part of the plan all along. A human, even with two final boss sisters, would have trouble taking over the world in his mind. So, by having her die, she would have to end up in Hell since her ambitions are not Heaven worthy, as he says. By killing her, he sends her on her way to become a demon, makes Deso fall to Hades as well to find her, and BAM now she can take over the world. He... just didn't think or know about the whole Prinny roadblock thing.
 * It still worked out in the end. She might be considered a Prinny, but she has neither the appearance nor the drawbacks of one, and thanks to being delusional and stubborn, she can unleash 100% of her full power without any backlash.

By the time of Disgaea 4 Father Salade has died and gone to Celestia
He's now working as the uniform designer for Angel Trainees...
 * It might have been earlier than Disgaea 4. La Pucelle has always had an uncertain chronology...

Valvatorez got the title of Tyrant from defeating Baal
Unless the Japanese is different, Baal is the only other demon to be called a Tyrant, and that's how it worked in the original Disgaea.
 * I believe there was a bonus boss in Prinny 2 who is also a Tyrant, and is seemingly unrelated to Baal.


 * I remember that that prinny is actually Baal's reincarnation.

God and Baal are somehow related, though how is uncertain, wheter it being the same entity in a different body or what not.
One of the more common forms for Baal's multiple bodies is that of an Enryngi. What does "God" manifest as? An Enryngi. With God Is Evil being in play in the 4th game, it may be possible.

Someone at NIS is a Castlevania fan.
The plot to late the game has some disturbing similarities to the back story of Symphony of the Night. Artina plays the role of Lisa, Valvatorez plays the role of Alucard, and Nemo plays the role of Dracula. To elaborate, the circumstances of Artina's death are in many ways similar to Lisa's (both die as a result of medicine related No Good Deed Goes Unpunished). Valvatorez is pissed but is stopped by the dying words of Artina. Nemo goes nuclear as a result, becoming a dark lord of sorts hell bent on the destruction and punishment of man kind as retribution for Artina's death.