Personally, I don't think that Black and White 2 are necessarily the end of Gen V. There could definitely be a Pokemon Gray coming eventually and Black and White 2, or at least the emergence of two cover legends instead of one, suggests that.
Resh, Zek, and Kyurem most likely all come from one dragon. In BW, we had three dragons: Reshiram, Zekrom, and Kyurem. In BW2, we have two Dragons: Black Kyurem and White Kyurem. In Gray, might we see the original Dragon? That's my take on it.
I think Hoenn will probably make an appearance in BW2, simply because of the things that were included in BW1's engine that really had little purpose. I think I might have mentioned this earlier, but why was Dive included in Black and White? To explore some pointless underwater ruins? Game Freak could have put the ruins in a cave in the sea and spent less time coding in Dive mechanics, but instead they chose to include an unnecessary HM move in the generation that finally moved away from HM moves. I think they included Dive so they could bring in Hoenn later since Hoenn is the only region to use Dive. Then, the BW2 logos have backgrounds that look like a ruby and a sapphire. BW1 ends with the main villain escaping and his son, now reformed, leaving with one of the dragons to a faraway land. If this is a direct sequel, which I bet that it is, then a Hoenn meetup of you, Ghetsis, and N doesn't seem so farfetch'd, now does it?
If the fusion of all three dragons is the strongest Dragon, then I would guess that it will have a BST of 720 like the strongest Pokemon does. For that reason, I suspect Black Kyurem and White Kyurem will have BSTs of 690, midway between regular Kyurem and Arceus. I would guess the type of these two Dragons to be Dragon/Ice (these are Kyurem formes, not Reshiram or Zekrom formes!) and the abilities to be Turboblaze and Teravolt. I would also speculate that the movepool of these two Dragons is the exact same as Kyurem's plus Cold Flare and Freeze Bolt, though Fusion Flare, Blue Flare, Fusion Bolt, and Bolt Strike are all logical additions.
As for the story, I'm thinking that Unova and Hoenn will be involved somehow, but not sure how exactly. It could be a "16 Badges" deal where you conquer the Unova League (with or without Alder, probably without) while fending off Plasma remnants then some cataclysmic event happens and you fly/take a boat to Hoenn and conquer its league for some reason while Ghetsis tracks down N's Dragon. The basic plot is that Ghetsis wants to fuse Kyurem, Reshiram, and Zekrom so you try to stop him. He gets Kyurem and tracks down N in Hoenn while you pick up the third Dragon somehow. Light, Dark, and God Stones are probably important too because you probably need them to fuse the Pokemon or something. Team Magma and Aqua probably try to wreak havoc too and you wreck their shit and catch Groudon/Kyogre (but not both) for the hell of it. Big battle happens and you win and Ghetsis and N escape again. Gray comes out in 2014 and finishes the story and unveils ReshiZekyurem in all its poorly named glory.
tl;dr: BW2 is part of a trilogy ending with Gray version, Ghetsis tries to recreate ReshiZekyurem but instead forms B/W Kyurem, you ally with N to stop him. Maybe 16 Badges too. Magma/Aqua probably come back too if there's Hoenn.
Or maybe this is just a crappy updated third version. Wouldn't that suck.