Hmm, well it would be a MMOFPS, hehehe, or something that also used bots...you take this large city, skyscrapers, subways, tunnels, get industrial area, factories, warehouses, homes, basically this large city, entirally detailed...then create an airport with another city....basically take a somewhat GTA type city, but where 99% of the buildings can be entered...
Anyway, then you create a person, and send them out to this massive world where they just plain battle it out with weapons, and objects around them, having a big enviroment would allow for some insane battles, and also allow people to interact with them all using their own type of play...for instance some mulitplayer levels in halo are very close knit, and don't allow for much play styles, you can't be a sniper and play a small level, as well as open levels make being someone who prefers up close battles difficult to play...this city would have close areas such as courtyards, streets, alleys, backyards, even open rooms inside some buildings...but then also people who love sniping can climb the tallest sky scraper, either using gadgets to climb it on the outside (watch out tho, snipers can also snipe you wile ur hanging from another building), or you could use the stairs or the elevator (but someone could get into the shaft and cut the belts and pulleys, or they could hack the computer and shut down the elevator (you would then have to exit the elevator, climb down the shaft and find another way up), then the snipers could find themselves their secret spots throughout the large city...
Plus with homes, you have tons of small rooms where you can turn lights off/on, hide, or warehouses where there are crates you can sneak around, or factories that are loud and you can't hear a thing of enemies, or factories that are so open you can't be queit and hide easily...
plus with the airport would allow for two cities...so you get tired of one city just go to the airport, fully rendered and detailed, and board a plan...go to the cockpit, and start it up...because it will be in the future, we'll say that the plane flys itslef to city B, and that will allow you and whoever else happens to be on the plane to battle it out on the plane...
now also, this big city would have tons of vehicles...cars in the garages, helicopters at hospitals, boats at the dock, tanks at the military compound, armored trucks at the bank...and all of these would have different controls, some with weapons, and others like the armored van could haul you and some friends around safely so that you could make attacks at another part of the city as a clan
and then there would be different game types, CTF, team battle, all for self, and city versus city...which would have you sign into one city, and ur city would try and overrun the other city...meaning you would have to load onto planes and pile out in each others city and destroy as much stuf in a given time, or simply kill all the opponents...then you could also zone off areas of the city so that you could have smaller battles in smaller levels...
oh yeah forgot the subway, hehehe, the subway would allow for nightvision goggles to be used, and you could also board the subs so that you could battle there, or walk through the tunnels with a rocket launcher watching as the train comes toward you, blow it up, getting 30 or so kills, or however were on the subway train...
which would be how scoring went on free for all, you would get ranked on amount of kills, and you would win after so many kills, considering that the servers and would be filled with people 250 would happen fast
obviously, technoglogy is not there for levels this big, nor are the size of servers, there would be 500 people in each city, nothing like we have ever seen...there would be no way for connection speeds or servers to allow that much continuas action without halt, or if that's even possible at all...but that would be the utmost in my opion...it wouldn't be an a living breathing city, no NPCs, just an empty city for battle
