I really don't think this idea is possible at all. Such a game would feel unfocused and very, very random. Further more the sheer amount of content you would have to generate to have all of those things in a single game would make it very difficult for a solo dev to complete.
Even just looking at the combat system I don't think it would work. It's like a doctor told me once when he was giving me my temporary leg brace to use before my custom ordered one came in: "This is one size fits no-one. It will work, but it won't be comfortable." I'm sure you could create a combat system that technically works for all of those different situations, but it would never work as good as a system designed specifically for that setting and genre.
I don't think you guys quite understand. My idea isn't doing all genres at once, it's just having the option of mixing genres. You could play a game that's a western alien invasion (Cowboys and Aliens), you could do cyberpunk far future (The Matrix), a d hundreds of other good combinations. You could do weird combinations that just don't make since, but at least there's the option.
Also I don't plan on each genre being too large. Most genres will have terrain, buildings, species, items, and at least 1 interesting game mechanic.
The combat system is like Dwarf Fortress. It keeps track of every body part (thus allowing randomized aliens), innards, and various injuries to those parts. I'm not saying a caveman could beat an alien, I'm just saying that battle is possible.
I've also thought of 4 more genres
Mafia : World and Theme
Vikings : World and Theme
Super Heroes : Theme
WWII : World and Theme