Due to complaints with my suggestions, I will bump this thread with new feature ideas.
Volcanic Wasteland: A large area that covers a couple tiles on the local embark menu. Mainly made up of flat magma with obsidian islands
Caldera: Only appears in areas of high volcanicness: Just a large, somewhat deep pit with igneous rock.
Hotspot: Rare, shallow pools of lava many z-levels down. Overtime, they bubble up. After many years, they may become magma pipes.
Natural Cliff cave: Appear on cliffs near ocean. May contain amphibious semimega beasts.
Monster pit: A mysterious glowing pit of ooze. Monsters come out of here. Only in evil areas.
Ancient shrine/ruins: Should I go on? They may/will be buried underground.
Glowing Orb: Another kind of HFS. Somewhere, many z-levels down is a glowing orb. It is just a 1*1 tile. However, gems will have a higher spawn rate as you get closer, so use that as a warmer/colder thing. If you dig it up, an evil spirit megabeast attacks your fortress, and will attempt to steal your glowing orb. If you successfully kill it, the orb will quadruple in value.
Ravine: High vegetation wedge in a cliff. May or may not have a river.
Hoodoo: Ravine with strange rock formations that are wider on the top then they are on the bottom.
Flowerpot Island: Tall, round islands.
Magma flow: Basically a river of magma.
Volcanic Shaft: Near volcanoes. Thin magma pipe that slants at an angle.
Great Tree: Huge, natural wooden tower. Dwarves can mine into it and build a fortress there.