Your environment needs to be complete 360 degrees around your character's start location and outward for 1 minute of travel. Outdoor scenes will need to use Unity terrain. The terrain must be sculpted and painted to match the environment model. If its a terrain scene, make sure you don't start near the terrain edge. The goal here is for the environment to feel resolved, at least to the level of detail and time we can afford.
Think about what your character is gathering or destroying and what travel paths take them around.
If you make an outdoor scene,make sure it runs well. Don't over-populate it with trees and grass.
The environment must thematically match your character.
Indoor or outdoor, you must model an environment "building" to match your character's theme or story. UFO's or bunkers are commonly done.
1024x1024 textures, many of these will be tiling, other will be non-tiling.
The environment must be given appropriate collision in Unity.
add lights to scene to communicate the mood of the scene and match any implied or explicit light sources.