@ Selnor
Adding splitscreen is not a major issue technically and it's not a default requirement in any case for a FPS at all. The much hailed Crysis doesn't support splitscreen at all, for example, neither does Bioshock which received game of the year awards.
Graphics-wise the same amount of pixels are being displayed, so there is loss in detail (so you could do with lesser quality assets as well).
The biggest problems actually relate to having a single player campaign designed for a single player experience specifically (for example the gorgeous 7.1 surround sound in Killzone 2), obviously killing enemies simultaneously changes the game dynamics. (too easy for example). So usually a seperate campaign needs to be developed. Gears of War was entirely styled for co-op, either with a bot or a player. As you know one of the things which makes Killzone 2 very impressive are the large number of buddy bots helping out with fighting the Helgast, however you have various parts in the game you fight by yourself or fight together with a different selection of buddies.







