There's very little question that the Primal power source is coming out in the PHB II, and whether or not a second power source will be unveiled with it I feel will largely dictate the composition of the book. As it stands now, I think we'll get...
3-5 Primal-powered classes
2-3 Divine-powered classes
1-2 Arcane-powered classes
0-1 Martial-powered class
Reasoning:
Martial Power will be long released, so more martial classes will lack future support and, therefore, be less desirable than other sorts of classes. That said, there's still no Martial Controller.
Arcane Power will be coming out the month after the PHB II, and it's fairly apparent that the Sorcerer is going to be in the PHB II. There are some questions, however, as to how WotC will handle the expansion of the Swordmage and Artificer: setting-specific books or any book related to Arcane characters. In the former, WotC might come out with two Arcane-powered classes in the PHB II to bulk-up the core arcane lineup for Arcane Power. In case of the latter, it's more likely that the Sorcerer will be the only Arcane class in the PHB II.
Divine Power comes out tentatively in July of next year, and it's likely that there will still only be two Divine-powered classes when it does unless they sneak one into Open Grave. As such, they'll likely want at least two Divine classes in the PHB II to get a better lineup. In fact, I'm predicting Inquisitor and Thaumaturge to be two new divinely-powered classes in the PHB II.
Finally, there's a chance of one or two power sources in the book. I think it will just be Primal, but it could be both Primal and Psionics. It's really just speculation, though, because WotC's plans beyond July of next year are shrouded in mystery.