Sorry, smoked the last of it for this list;
Off the top of my head, in terms of setting and style (not necessarily map layout);
- A Forerunner 'archive', a greenhouse-like series of enclosures with bizarre plant species surrounding a central Epitaph-style hall. 'Slides' containing various life-forms are constantly whizzing in and out of the room via mechanical claws and ceiling-mounted tracks, and you can shove other players into them for damage.
- A Covenant listening post on a jungle planet. A massive tower sitting in a festering marsh. The lower areas are caked with mud and flooded while the upper areas are covered in vines and growth. A huge gas planet hangs overhead, giving everything a deep blue tinge that highlights the green forest and purple structure.
- A human research facility in the arctic, set up like an oil platform. Huge waves crash up and over the deck (can push players off the stage) and there's a layer of ice on everything; chains, walkways, windows, everything. Beneath the upper deck and structures are small labs and maintenance hangars. When the waves crash, the whole level rocks back and forth and the labs hanging below the deck swing wildly. Layout very similar to Lockout.
- An underwater Forerunner structure similar to parts of Halo 2's Regret. Large portions of the map are flooded, so there's waterfalls and stuff where different parts of the base connect. Lots of glass ceilings and tubes that you can see other portions of the level from, as well as strange alien critters. Like you're crawling through a sewer made of glass.
- A BTB map in a wetland, but well-lit at sunset rather than the usual murky affair. Lots of low-hanging trees and mud everywhere, so when you go tearing around in a warthog it feels like you'd be better off in a boat. Have suspended foot bridges or wooden walkways, and lots of high grass to hide in, so infantry actually has a chance to evade vehicles. Maybe some concrete bunkers too.
- A battle on the hull of a Covenant cruiser. Skybox is built to make it feel like you're fighting to board the craft in the middle of a large space battle (Sabres and banshees whizzing by, strafing the hull occasionally). Big turrets firing off into the battle, etc. You should feel like you're moving around insanely fast.
- Would love to see a map use the atmosphere of ODST: Dark and rainy, destroyed and depressing. Lit with flashes of lightning. Something urban-feeling like the lovechild of Turf (narrow alleys, building-to-building combat) and Headlong (room for vehicles, lots of height and platforms).
- Wouldn't mind seeing a Covenant factory with banshees or weapon pods being constructed all around you. Could be Colossus or Longest-like in terms of the conveyor belts and lifts and such. Lots of anti-gravity devices pushing components together, etc. The map itself could take place on walkways and light-bridges that can be switch on and off, leading to some interesting CTF matches with different routes being player-activated.