At one time, we were very short on new theme suggestions, and Elemental Planes Stuff was brought back from the orphan themes list for a 2nd chance (it originally got 0 votes in the poll for May 2011).
This would be a great month for reskins of existing creatures (to make them better fit air/earth/fire/water elemental planes), perhaps with added emitters (fire), crystals or rocks (earth), transparency (air), or txi texture animations (water).
This could also be a great month for exotic elemental planes themed tilesets. Maybe a City of Brass set with metallic buildings (could be a reskin of something else), islands of rock within the planes of fire, water, or air (and marked with strong traits of the surrounding element). And such tilesets would go great with new elemental plane skyboxes (or skyspheres that work better with unlocked camera angles).
Placeables that fit the elemental environments would be great, too. Columns that ripple/shimmer with glowing light like embers in a fire, walls that look like flowing water of various shades, subtle placeable VFXs to show currents under the water or in the air, giant "bubbles" to surround pockets of breathable air the plane of water, new fire/lava placeables that give off dynamic light (for illuminations, shadows, etc.).
Onto item ideas... How about earth elemental armor that uses OldTimeRadio's plt + tga trick (plus self-illumination) to add glowing crystals, glowing (but not flaming) metal spears (for the salamanders released a while back), air/water weapons with built in trailing effects or txi files to make them mostly transparent, crystal-edged blades, solid stone bludgeoning weapons, new ioun stones, armor with fainty glowing (self-illuminated) edges in various elemental colors (orange, green, blue, white, whatever).
On the scripting side, the elemental planes are foreign places with their own unique environmental effects, be it on movement, breathing requirements, damaging energy (heat), or even changes to the effects of spells. There's also room for strange quasi-magical radiations from crystals, stones, magical flames, magical pools/fountains, and more that could have uniquely scripted effects. And do you dare eat what's considered by the denizens of the City of Brass to be a "hot pepper"?
And don't forget prefabs. This theme is a fertile field for those that want to use existing content to create something small scale (a single item left behind by a slain earth elemental) to large scale (perhaps an entire colony of halflings that started as a small group of pilgrims sucked into a portal to the elemental plane of water, only to settle on an island paradise within a permanent air bubble).
Yeah, just throwing out some ideas to get everyone's creative juices flowing, since the original theme description is kind of sparse.