Devil Survivor/WMG

One party member in De Su 2 will be a Kuzunoha clan member
Possibly the 4th of the great summoner clans (It's been mentioned there are 4, but we only know of Raidou, Geiran and Kyouji). Why? It would be awesome!

Cain didn't kill Abel out of jealousy per se...
Maybe it turns out that God really is (or was, perhaps He has mellowed out somewhat in the present-day) kind of a douche. Demons most likely existed back then, and it goes without saying that the Bel demons were among them. So maybe He wanted to use Abel (you know, A Bel) to bring the demons under control and/or destroy them all. Cain caught whiff of this plan so he killed his brother to protect him.
 * Done and done. Thanks to a certain Louis.

Black Frost's "Warrior of Love and Rage" comment is a reference to Green Day
"I'm the son of Rage and Love, the Jesus of Suburbia....."

The King of Bel was part of one of Cain's previous Rage Against the Heavens.
He's been reincarnating all this time; surely the events of the game weren't the first time he attempted to lash out against God. He supported the King from behind the scenes, building him up into an entity that could challenge God and His angels. That didn't work out so well, but Cain isn't the type to give up.

Cain is an Unwitting Pawn in an Eternal Recurrance plot.
The real reason he contantly reincarnates is to serve as a mostly human adversary -- or rather, a symbolic representation of humanity's worst flaws. This 'justifies' the angels' Holier Than Thou attitude and gives them an excuse to keep punishing humankind and plotting to take away pesky little things like their free will.

Cain has long since figured out his status as a pawn; however, that just fuels his determination to succeed. He may have attempted to not play along in the past, to live a normal life and avoid dragging any loved ones he might have had in his current incarnation into the fray... only to see them killed or worse as part of his eternal punishment. After all, his being Cain is all the justification the angels need to act against him... Which, of course, only fuels his desire to Rage Against the Heavens, even knowing that it only plays into their plotting.

One of the above theories about Cain will be proven true in Overclocked.
Maybe the 8th day of Amane's or Naoya's routes.

There will be an "MC" route rather than the ones that are offered by other characters.
Though I highly doubt it, there's still a possibility. Maybe it will be a bonus route to be unlocked after finishing all the other ones. In this route, everyone will join your party.

Mari Mochizuki is the incarnation of Nyx.
Just like her child, Thanatos, or, she has a black hair. And she's high. Nyx/Mari no longer has the power over Death/Demon/Shadow because, you know,

Aya is a reference to the author of the original Digital Devil Story novel series.
Aya Nishitani's trilogy of books started the video game franchise, even if only the very first Megami Tensei game had any real connection. In Devil Survivor, Aya was able to summon demons with her song, and the Demon Summoning Program uses that melody. She helped to author the program, and thus the following lock down of Tokyo and the game's plot. She was also pulled into the demons' world. Similarly, Aya Nishitani helped to start the video game franchise, but there is little information on her after that, sort of like she vanished.

Zatch/Gash Bell is legible to participate as the next King of Bel
One, he's a demon child, commonly referred to as a mamodo/monono. Second, he's a powerful thunder-burping demon. Third, he manages to beat every obstacle that comes his way, from millennium old petrified victims to insane megalomaniacs. Fourth,  Fifth,   And finally, the most obvious bit; his name is Zatch Bell (or Gash Bell if going to the original Japanese; point still stands.)

Will Smith is a Bel
He is, after all, the Prince of Bel-Air.