you can describe literally every genre on the planet that way
The vast majority of FPS do not do anything outside of Call of Duty + gimmick X
The vast majority of TPS do not do anything outside of Gears of War + gimmick X
The vast majority of platformers do not do anything outside of Mario + gimmick X
The vast majority of 4X games do not do anything outside of Civilization + gimmick X
The vast majority of RTS games do not do anything outside of Warcraft + gimmick X
The vast majority of JRPGs do not do anything outside of Final Fantasy or Dragon Quest + gimmick X
The vast majority of sports games do not do anything outside of Madden + gimmick X
The vast majority of character action games do not do anything outside of God of War + gimmick X
Welcome to the concept of game genres.
However the core mechanics of MMO's have rapidly fell out of favor as evidence by the depleted MMO audience/market. So adding a twist to a maligned core design is less meaningful. This creates a lot of games that people would call mediocre or bad and the genre becomes easy to shrug off by those that feel the core design isn't strong enough to be a foundation for minor gameplay gimmicks.