I'm playing with the idea of introducing Wuxia -- really Quinggong, the Chinese flying and jumping thing (popularized by the
wire fu of many films, such as The Matrix). Most of Caldera's streets are only 5 feet wide, and my imagination immediately jumps to PCs bouncing up the close walls, back and forth. I reimagine the Eladrin fey step as an advanced leap, not a teleport. What if all PCs had this ability to some extent, letting them jump upward along wall faces as regular movement? Caldera has some pretty unique terrain and I want to encourage players to take advantage of it.
A lot of my ideas from Metrocalypse are getting recycled, too. In Metrocalypse, a real-world city gets sucked into a savage D&D world. Normal, everyday people start transforming and exhibiting magical powers. This is a justification for races and classes. In Caldera, many of the races and classes already existed, but the city's magic turned up the volume and flooded the world with monsters. I'm tempted to say that Caldera's world has a limited number of fantasy races, and the rest are magical mutations. That way, I don't have to create a government and culture for a race that I'd just as soon eliminate from the setting, and I don't have to tell a player they can't play a gnome, because normally I hate them; oddly, though, Caldera seems to wants 4E gnomes and 3E gnomes, so we'll see what happens.
I'm creating a new character race, Gargoyle. It subsumes the role of Goliaths (MM2), Warforged (Eberron), and Minotaurs (Dragon #369). They're literally city statues come to life, and they always have a strong bond with their home neighborhood. There will be a small, medium, and large racial option. The small gargoyle gets a limited version of stone form (see MM2), which confers minor damage resistance and regeneration. The medium gargoyle gets goring charge (see Minotaur). The large gargoyle gets the normal Goliath abilities.
A cool idea about elves fighting their savage, horse nomad roots. Choose either Eladrin or Elf as your race. Whenever you retrain between levels, switch if you want! Swaps all your racial bonuses, abilities, powers, etc.