Kuma showing up is probably just to let Luffy know that Ace got fuck pwned by Blackbeard and to send the Mugiwara crew after Blackbeard, hoping to cancel the World Government's two biggest problems out in one shot. He might also be there to put an end to Moria himself, who has clearly violated the rules of Shichibukai-dom. I'm pretty sure he's not gonna offer Luffy a Shichibukai position, not after Luffy totally trashed Enies Lobby.
BTW, has Oda confirmed that Lucci is dead? If so, he'll be the second villain to actually DIE, after Arlong. L O L
Vagabond, One Piece has some pretty dark bits. Robin's backstory is probably darker than every dark moment in Naruto combined.