Disgaea 4/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...

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!

Valbatoze 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.
 * Well, Valbatoze and the Netherworld President Hagos do seem to know each other, but they don't seem to have the same connection that Laharl, Adell, and Mao have with Krichevskoy,, and the Dean.

Valbatoze will be voiced by Yuri Lowenthal.
He looks the part...
 * Most people are saying Johnny Yong Bosch due to the similarities to Lelouch.
 * Yuri Lowenthal would still make sense then, being the English voice actor of Lelouch's best friend/arch nemesis, Suzaku

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.