Thinking about what some of the classes we can expect to see in the months after 4E becomes available... This is all crazy speculation.
Fairly Confident
Bard: Arcane Leader. Seems pretty clear; Arcane is the closest power source fit unless you want to invent an entirely new music-based power source, and the Leader role seems pretty clear.
Monk: Martial Controller. We know it's not Striker, and no class could be more martial. Give him lost of throws, grapples, stunning attacks, nerve strikes, trips, etc. and he's the guy in charge of who's doing what where on the battlefield.
Swordmage: Arcane Defender. I don't see how to make him a Striker without stepping all over the ranger and rogue. On the other hand, mixing magic in with melee prowess should make an excellent defender.
Barbarian: Primal Striker. Someone needs to be based off the same power source as Druids and Barbarians are a natural first choice. I can see this class getting a ton of HP and some kind of shrug-off-status-effects ability to compensate for poor defenses.
Minimally Confident
Druid: Primal Controller. Shapeshifting, summoning, healing, and utility nature magic is way too much to be all given to one class. With iconic spells like Entangle, Stone Shape, and carefully chosen summonings, the Druid would make a great controller--and perfect for an underrepresented role. Healing and major combat shapeshifting would probably have to go to another class.
Assassin: Never appearing as a core class. The lone figure who's deadly only when unseen doesn't fit into a party well at all. I can easily imagine a paragon/epic path granting shadowy hiddenness and massive bonuses on sneak attacks to rogues, though. If this ends up as a core class I would expect it to be based off of a new power source like "Shadow" or something.
Psion: Psionic Controller. You could also develop the class as a striker but I suspect there will be plenty of strikers. There's also a chance on "leader"; I can imagine a psion telepathically coordinating the battle, giving orders, telling the cleric to duck just in time, etc.
Psychic Warrior: Psionic Defender. The biggest problem will be differentiating it from the swordmage.
Illusionist/Enchanter/Beguiler: Arcane Controller. The demand for illusionists is going to be high and the beguiler class worked pretty well. Oddly enough, despite the same power source/role as the Wizard, this should be very easy to differentiate. The abilities would be focused on charm/compulsion/misdirection, thwarting enemy attacks through deception and trickery rather than physical barriers like the wizard throws down.
Wild Guesses
Shaman: Primal Leader. Think of the 3E druid as a pure spellcaster: healing, buffing, throwing down lightning and fire, maybe a summon or two. There have been no hints at the existence of this class but I think it would work well.
Favored Soul: Divine Striker. If the Favored Soul idea makes it to 4E you could do pretty much anything with those powers. But the Divine Striker is a niche waiting to be filled, and the Favored Soul could fill it with a combination of offensive magic and self-buffed melee combat.
Shugenja: Divine Striker. If they decide to go for an eastern flavor, this divine elementalist could work like a holy warlock.
Necromancer: Arcane Defender? You gotta have necromancers, but you could design this class in any direction you wanted--zombies as meat shields (defender), soul slayer extraordinaire (striker), drain your enemies' power and give it to your friends (leader), grasping hands from underneath the earth (controller).
Sorcerer: Arcane Striker? I have no idea how to differentiate this class from the wizard and warlock.
Fairly Confident
Bard: Arcane Leader. Seems pretty clear; Arcane is the closest power source fit unless you want to invent an entirely new music-based power source, and the Leader role seems pretty clear.
Monk: Martial Controller. We know it's not Striker, and no class could be more martial. Give him lost of throws, grapples, stunning attacks, nerve strikes, trips, etc. and he's the guy in charge of who's doing what where on the battlefield.
Swordmage: Arcane Defender. I don't see how to make him a Striker without stepping all over the ranger and rogue. On the other hand, mixing magic in with melee prowess should make an excellent defender.
Barbarian: Primal Striker. Someone needs to be based off the same power source as Druids and Barbarians are a natural first choice. I can see this class getting a ton of HP and some kind of shrug-off-status-effects ability to compensate for poor defenses.
Minimally Confident
Druid: Primal Controller. Shapeshifting, summoning, healing, and utility nature magic is way too much to be all given to one class. With iconic spells like Entangle, Stone Shape, and carefully chosen summonings, the Druid would make a great controller--and perfect for an underrepresented role. Healing and major combat shapeshifting would probably have to go to another class.
Assassin: Never appearing as a core class. The lone figure who's deadly only when unseen doesn't fit into a party well at all. I can easily imagine a paragon/epic path granting shadowy hiddenness and massive bonuses on sneak attacks to rogues, though. If this ends up as a core class I would expect it to be based off of a new power source like "Shadow" or something.
Psion: Psionic Controller. You could also develop the class as a striker but I suspect there will be plenty of strikers. There's also a chance on "leader"; I can imagine a psion telepathically coordinating the battle, giving orders, telling the cleric to duck just in time, etc.
Psychic Warrior: Psionic Defender. The biggest problem will be differentiating it from the swordmage.
Illusionist/Enchanter/Beguiler: Arcane Controller. The demand for illusionists is going to be high and the beguiler class worked pretty well. Oddly enough, despite the same power source/role as the Wizard, this should be very easy to differentiate. The abilities would be focused on charm/compulsion/misdirection, thwarting enemy attacks through deception and trickery rather than physical barriers like the wizard throws down.
Wild Guesses
Shaman: Primal Leader. Think of the 3E druid as a pure spellcaster: healing, buffing, throwing down lightning and fire, maybe a summon or two. There have been no hints at the existence of this class but I think it would work well.
Favored Soul: Divine Striker. If the Favored Soul idea makes it to 4E you could do pretty much anything with those powers. But the Divine Striker is a niche waiting to be filled, and the Favored Soul could fill it with a combination of offensive magic and self-buffed melee combat.
Shugenja: Divine Striker. If they decide to go for an eastern flavor, this divine elementalist could work like a holy warlock.
Necromancer: Arcane Defender? You gotta have necromancers, but you could design this class in any direction you wanted--zombies as meat shields (defender), soul slayer extraordinaire (striker), drain your enemies' power and give it to your friends (leader), grasping hands from underneath the earth (controller).
Sorcerer: Arcane Striker? I have no idea how to differentiate this class from the wizard and warlock.