There should be an addon that allows you to designate a specific area or the whole entire level as a “biome”. For example, if it is set to tropical, vast jungles, and lots of waterfalls and other things like watery lakes and animals are made. If it is set to desert, the reverse happens, making the area appear sandy with lots of cacti and sand-eroded rock formations, and very few oases with palm trees and underground clay and rock houses surrounding some oases. If it is set to snow, the whole area is covered with what appears to be snow, and conifer trees and lakes with ice on top and log cabins are made. If it is set to marshland, mushrooms and goopy mucky terrain generate, with some mangroves and a lot of moss and mud. If it is set to ocean, large amounts of water generate, among with little islands, and undersea plants and marine life.
Well, you have to program that. You have to model the trees, animals, etc.
Haha…and there should be a button in the engine that says “generate great next-gen game, that sales well and will make me a fortune”. 
You basically ask for a world generator…it is possible but the outcome in every engine(not only bge) will not be what you imagine.
