On GR2 we had Siege, but fans wanted more bases. So, we added Double Siege. Some people liked that, but they wanted more randomness. So, we added Blind Siege, etc, etc. Honestly, that was a lot of work! I thought to myself "Why do we keep making these over and over again, when we can just let players do it themselves?!?"
So, we came up with a system of options and defaults that lets players define their own game modes. An important design pre-req for this was that we couldn't have a system that was clunky to use, or alienated people that liked the old systems, so what we ended up doing was building a whole bunch of options, broken down into categories of elimination (kill everyone), territory (capture zones), and objective (control objectives). Then, within each mode, we added customization options that fit within the overall scope. Next, with all these options available, we built "presets" that made up the gametypes from past games, like Siege and Sharpshooter. This way, if people don't want to deal with the customization, they can just pick a pre-set, or if they do, they can get down into the nitty-gritty of the system.