Just from your first paragraph, I would say, don’t look into landscapes 
Large landscapes are really going to eat a chunk out of the frame budget, and if the player never goes near it, it’s a waste.
I’d recommend finding out about HDRIs.
You can get amazing ones for free, you can buy them, you can make them in the engine easily. You can use a decent HDR to make it look like there’s a huge landscape surrounding the player, at virtually no cost.
Here’s a quick intro
