Maps with natural areas that aren't rock-based.

[baja blast rd.]

Obviously Doom's architecture is better suited to depicting rocky stuff: caves and canyons and the like. Because slopes and 3D stuff and all that jazz are not possible. I'm looking for maps that manage to create convincing and attractive plains and forests -- grass-based terrain, basically -- in spite of that. Any suggestions?

[Rayziik]

Hellbound MAP07 comes to mind. It isn't named "The Woods" for nothing, although the gameplay and layout are pretty grindy/linear.

[Varis Alpha]

The Ultimate Doomer's wads generally has at least one level with grassy/forest stuff in them. Super Sonic Doom has that with map 2 through 4, Serpent: Resurrection has a large grassy first hub-world, License to Spell Doom has a small grassy area in map 6 but isn't the main focus, Fragport's map 07, and maybe some others. note that everything sans Fragport is ZDoom-based though, and whether you like them or not it depends.

[Cynical]

Try Deus Vult II map 03.