Assuming they're relying on other books for the Swordmage (FRPHB, we already know) and the Bard, and their minimum ideal for putting out a power source sourcebook is three classes in the source (as it is with Martial Power), I suppose they could still squeeze in Psionics. In keeping with my other thoughts above, then, we'd be looking at 3 Primal, 3 Psionic, 1 Divine and 1 Arcane. Three classes would seem to be the minimum to complete a set sufficiently for them to be willing to release a Power sourcebook. That puts us in an odd place where, although in the core books there would be three to a source for each, including all reference sources arcane would have 5 (perhaps 6 if artificer is fully fleshed out in Dragon) classes by the time PHB2 is out. That all but cements Arcane Power as the second power sourcebook.
Arcane: Sorcerer
Divine: 1 unknown.
Primal: Barbarian, Druid, Shaman
Psionic: 3 unknown.
I, R, T, and W.
I really don't think "I" is Illusionist. One, we've seen it before. Two, they're now going to include some illusion-based power options for Wizards to fill that gap.
Of Psionic, Telepath does seem likely.
I think the most 4e friendly psionic specialties are probably: Telepathy, Psychoportation, Psychokinesis, and Psychometabolism. They've effectively relegated ESP-like powers to rituals, and put in the "warning blurbs" for DMs that have PCs that want to attempt scrying, so I can't see them basing a whole class around Clairsentience. Summoning still doesn't appear to be in elsewhere, so it's unlikely Metacreativity would be the first. You could probably fold metabolism and teleportation powers together to make one viable defender class with a splash of striker mobility, with the flavor that their control over their bodies allows them to move in unconventional ways.
Wasn't there a short three or five level prestige class somewhere, at one point, for Psionic Nomads called the Wayfarer's Guild? Or am I imagining that? Wayfarer (or perhaps Wanderer) is a stretch, but that would be a 'W'.