In order from what I feel is Most Likely to total longshots:
1. Psionics. We've pretty much been just out-and-out told that it's in. I'm guessing it'll be in its own book, rather than the PHB2, but it could go either way. Personally I'm hoping for its own book, because that way we'll get lots of classes instead of just 3 or 4 and all the cool psionic races.
2 & 3: The Shadowfell and the Feywild. I'm covering these two together because my justification is the same. They have a vested interest in integrating their new cosmology into the game, and the two "near planes" each allow for some fairly iconic fantasy categories of magic.
The shadowfell power source could include the Necromancer class that has been mentioned, other cool undeath sorts of themes, and also most of the fluff concepts from the shadow-magic section of Tome of Magic, which I think many people can agree was some pretty cool fluff, even if you didn't like the mechanical implementation of that magic system. Several more classes could come out of the concept, a shadowblade striker, something like the shadowsmith fleshed out into a defender class, undead or shadow-creature minions, and they could use the coverage as an excuse to make their new pets the Shadar-Kai into a PC race.
As for the Feywild, there's a lot of nature-magic and spirit-magic from a variety of myths/genres that could be modeled. Bards and Druids are both suspiciously absent from preview material, and could both be worked into this concept very easily. Heck, 3.5 druids could be split into an environment and weather-based controller class, a healing and pet-based leader class, and a shapeshifting/animal-totem following defender class pretty easily. The Illusionist class that has been mentioned could fit in with this powersource, too, although that's more likely to be Arcane.
As with the shadowfell, there could be associated races, too. Elves and eladrin would obviously be involved, but it doesn't have to stop there. Fey critters and animal-people are both consistently popular, so I wouldn't be surprised by races like satyrs and pixies or just fey-blooded mortals and cat-folk, bird-folk, bear-folk, and all the other things that would sell books to the Furries.
4: Artifice. I think this is still pretty likely, but might just get folded into the Arcane magic source instead. Also, it's more likely that this'll be part of the Eberron Campaign Setting than canonized in the PHB2 (or 3, or whatever). But still, artifice-style magitech could support several character class concepts above and beyond the "magic item vending machine, plus wand-blasting cheapness" we got in 3.5. How about a striker with a big alchemical blaster or energy-based raygun? How about a Defender who's constantly enchanting and upgrading his magical armor until at high levels he's friggin' Iron Man? How about an alchemist-grenadier Controller who can sling explosions and poison gas clouds around the battlefield as long as you keep her supplied with eye of newt and otyugh oil? I think we're gonna see SOME version of this, but I'll probably be surprised by exactly what.
As for associated races, well, warforged, duh.
5. Dragonblood. In 3.5, we've got the Sorcerer, kinda a controller, the Dragon Shaman, clearly a leader, the Dragonfire Adept, pretty obviously a Striker, and we could have a Defender too if they show a little love to the Dragon Disciple, which I always thought was such a cool idea but really strangely executed in 3.5. Prestige Class about turning into a melee monster with spellcasting prereqs? Mimicking a template instead of giving unique features? As a base-class, they could combine monk-like flavor with slowly growing claws and scales and wings and stuff, and who doesn't love that? Even if they don't go for that, there's a ton of dragon-dusted PrC's in Draconomicon, Dragon Magic, and
The huge book of Kobolds Races of the Dragon. They could cobble together a class or two from all those ideas, surely.
Races for this one would be pretty easy, too. We've got multiple varieties of Kobolds over the years, dragonborn, spellscales, even lizardfolk. Also, if they want to make me do the happy-dance, they could bring back Urds.
6. Elements. This would round out the new cosmology with #'s 2 and 3, and there's a seam to be mined there, at least. It'd probably be easier to just fold it in with Divine magic, though. Races for this one could be cool: who doesn't miss the Genasi races?
The stuff I don't really expect to see:
Truenaming. Awesome concept, too fiddly, and now there's a whole lot of haters out there.
Incarnum. Would be cool, but kind of obsolete now that Vancian casting is dead.
Far Realms/Chaos magic. Call of Ctulhu sort of stuff would be cool, but isn't family-friendly or widely accepted. Still, I do love me some tentacles, and I'm not the only one.
EDIT: As for Ki, I hope they don't bother. Just leave martial arts in the Martial power source. It's not like they ever tried very hard to be loyal to the real legendry and fantasy-tropes of martial-arts fantasy and wuxia, or ever wrote any compelling and cool mechanics for internal or mystical martial arts. Not even in Oriental Adventures, man. Just my opinion, but they should stick to their strengths and not take another pass at bad stereotypes and weird collections of powers that don't have anything to do with each other.
