So I started looking through my old AD&D 2e books today, just for kicks, and one of the things I came across that I had always thought was cool was the Character Kits from the Skills and Powers book. Now, I know the Player's Option books are pretty much universally derided as being too prone to power-gamer min/max abuse, but I don't really care. I was always more interested in the story possibilities inherent in each of the kits, and I think they can be exploited to good effect in 4e.
In Dark Sun, we were introduced to the concept of Character Themes, and the Skills and Powers Kits fit the bill quite well for more homebrewed Themes. Obviously, themes should be appropriate for the world they come from. For instance, in my Islands in the Sky campaign, the first theme I would want to introduce would be something concerning the airships. Just looking at the Players Option list, this opens up the following: Mariner, Pirate, Smuggler and Swashbuckler. These kits all speak to possibilities inherent in the nature of airships.
A lot of the kits seem like they could very easily fit into any campaign. Things like Merchant, Peasant Hero, Rider, Outlaw, and Spy would be common character types.
A number of these kits have become their own class in 4e! Assassin and Barbarian are the most obvious. The Cavalier is an Essentials Paladin build. Gladiator is already a theme in Dark Sun. Sharpshooter is basically a ranger build, and Animal Master is basically the Essentials Druid Sentinel.
Now to start coming up with Encounter powers for these new themes... The original kits only gave players a bunch of suggestive fluff for their character, a benefit and a hindrance. I guess the idea was that the kits, if allowed in a game, would balance themselves out by having a benefit and a hindrance. Regardless, it's going to take some creative thinking in a lot of cases to come up with some encounter powers to give these themes!