After playing gears of war 3 for about twelve hours over the last few days, Rykuta and I have decided that we must make our planned zombie survival in that style.
For people that have not seen the new horde mode:
You are a team of humans fighting off waves of enemies of increasing difficulty. You are able to build defenses, buy better weapons, and upgrade fortifications. You do all these things through a cash based system where you get money for killing enemies and completing challenges, and spend the money on whatever you like. You can also level up skills such and make your money go to better use!