You can't change it much without staying true to the type of game.
This is an astronomically stupid statement.
Let's take a look at another popular FPS series, like Battlefield. Ignoring the spinoffs Vietnam, 2142, and Bad Company.
The franchise started out in 2002 with Battlefield 1942, a team-based MP only shooter set during World War II. The main objective on each map was to capture command posts and drain the enemy's respawn tickets until they ran out. Most, if not all maps had several vehicle spawns, from jeeps and tanks to battleships and divebombers. Players could also pick from several different classes each time they respawned, to deal with changing situations.
In 2005, DICE released Battlefield 2, the "sequel" to 1942 (or as close as a sequel gets for a multiplayer only shooter), and is widely considered the best in the series. Core gameplay was mostly similar: Capture command posts, drain the enemy tickets. But this time, it was set in the modern era, and with the time period changed, many other gameplay elements were too. Most notably, the introduction of the commander position, who got a birds-eye view of the whole map and was able to direct the team using information they might not know, as well as provide support in the form of vehicle drops, artillery strikes, UAVs, and supply crates. In addition to the commander, the Squad mechanic was introduced. Each squad leader would receive orders directly from the commander to attack or defend certain locations. Other commands such as mining an area or repairing a strategic asset existed too but attack/defend/move were the most commonly used. The classes also got changed up a little by allowing players to unlock new primary weapons, like an automatic shotgun for the anti-tank class (who normally gets an SMG), or a MP7 for the engineer (who normally gets a shotgun).
Now, in 2011, Battlefield 3 is on the horizon. Again, core gameplay seems to be the same as always, with the assimilation of the "Rush" mode from Bad Company. It has simplified the amount of classed from seven down to four. However, these four classes are capable of more flexibility than the seven in Battlefield 2 (engineer and anti-tank were combined, so were assault and medic). The commander position seems to have been removed, but the squad mechanic still remains. Vehicles are still there (as always) only now the maps are destructible, which could mean that vehicles could be used for more than simply killing the enemy. Unlocks appear to be there too, but with much more variety than just selecting your primary weapon. Several other new minor mechanics are introduced, such as limited health regeneration and the ability to mount your LMG on a surface for more accurate fire.
So yeah. It is possible to innovate and try new things while still staying true to the core gameplay aspects. Releasing the same game every loving year is not innovation.
