Hmmm, I was originally going with Arima being the current "OEK" that Eto wants killed. I've been reading some theories suggesting that Arima is either a descendant or had a portion of the original OEK implanted into him at a young age -- the Washuu/V would have then brainwashed him into becoming a "puppet" for them as Shachi stated, thus keeping the OEK of this generation under their thumb in order to prevent an imbalance between the human/ghoul world. If this were true, and Ken kills Arima, he would in a literal sense become the "The Second King" (coincidentally the title of chapter 56 after he's defeated Eto) of the current generation. This is provided he actually takes up the title of course.
But after reading some more, this theory doesn't make much sense when we consider that V has sent Furuta out on a wild goose-chase trying to dig up information on who the OEK may be for years now. There'd be no reason to have him waste time infiltrating all of these other organizations if they already had the OEK under their control.
If there's one thing I'm willing to bet on, it's that Kaneki is going to usurp the current king and become their successor (you either become a king by birthright or by forcibly taking over the throne). As to who the current king is, no idea. The likelihood that it's someone we've never seen before is low since Eto mentioned that the king is "in your bellies", which I assume means that the current king walks among them.
Anybody else catch the bead of sweat running down the side of Kaneki's face in the last panel he showed up at? He must be really nervous. Or really hot, he should ask Kanou for one of those nifty Hawaiian shirts.