For example, Duke Nukem 3D's mapping scene allowed a lot of different types of levels, from cities to deserts to bases to alien worlds and all sorts of other environments just with the base set of content, without needing any extra textures and stuff.
How much variety can Half-Life 1 provide using its base set of content? I'm looking to make a city-based level, but most HL1 is mostly set in labs and deserts and alien worlds so I don't see much from for city-based content such as building textures, road textures, skybox, etc. Not to mention it has a troublesome Quake-based map editor, which means levels can't be too big.