I can help more if I had specifics, like what type of gameplay and environment you are thinking of. Feel free to reference other games for comparison.
A general approach I would take is to identify what assets are going to make up the bulk of your environment and tackle them first. Start with developing a small area that represents the larger scope in your game. For example, if were developing a game that took place in a futuristic city, I would not start by making a massive city. I would make one block or street instead.
Inside this scene, it would break it down to common elements that I can reuse to construct a city. While you are doing this, your first time should be spent on the things that affect gameplay. For our hypothetical game, lets say its GTA on a Mars in the year 3000.
I don’t think you’d want any assets to have too much detail and a basic texture/color. You want to concentrate on strictly form. I’d make a simple roadpiece, probably with a sidewalk built in. Then I would make a number of interesting shapes to represent buildings. It’s the future so they are funkier than normal. I’m not even doing interiors because that makes it a hell of lot easier. Now you can start building your city block in the world. If at this point you are still having problems determining what to make for pieces, limit it by scale. Do not make anything that would fit into a 20x20 foot box (for this example, for a different type of game that would be different, be flexible. A flight sim would be 200ft. So no signs, cars, benches, robots, trash cans, - any smaller detail.)
Run around your world, make adjustments on this scale. You will start to notice what you need. Bridges maybe? Overpasses? Address these larger concerns and then move onto the next scale level. Make anything that is between 10 and 20 feet. Cars, light poles, oxygen generators, larger props. Place them around. Feel out your environment.
At this point you could do a test. Duplicate your city block. Rotate it 90 degrees, place the duplicate next your original block. Delete some buildings from one block. Rotate some others. Copy these two blocks, rotate 90 and place adjacent to the previous set. Switch up the buildings in those blocks. Now you have a 2x2 section. Play your environment. How does it look? What is repeating? What is working well to sell your vision?
Looking good? Move to the next level of detail. Street signs, curbs, building details - maybe make a kit of windows and doors your can tack onto your building shapes, space dumpsters, etc.
Rinse repeat. You can stop at any point, at which you should have a collection of assets that can make a city block. You eventual goal is to replace these temp assets with ones that represent the style and look of your game. You could work on that now, or you might want to do some planning on paper, what kind of limits you want to go for, what are some landmarks you might want. Maybe the city has a big dome over it. Or a giant generator in the middle. Again, stay general. Take that information and start to build your whole city. With that guide you can take your block, rotate it, change it up, clone it, clone that set, and the next one, etc. Mockup your landmarks and start putting them in.
Hope this helps - any questions?