To stand out in the world of bullet hell shooters, developers have tried many numerous gimmicks, mechanics, and overall interesting experiments. For instance, in the above video, the game IKARUGA by a small Japanese studio called Treasure, you must alternate between “yin” states and “yang” states, and while you can absorb bullets of your color, you cannot absorb bullets of the other color. Also, you deal double damage to opponents of the opposing color, creating an interesting risk vs reward dynamic where the player must make split second decisions on whether to kill off an enemy quickly or attempt to dodge an onslaught of opposite colored bullets.
Also, in games such as the Touhou franchise, the shooters are known for changing up mechanics constantly. For instance, in TOUHOU 13, the main gimmick is “Life after Death”, and after dying you have 10 seconds to attempt to defeat whatever killed you to regain your life. This franchise is one of the more successful bullet hell franchises, and its fans have come to expect new and interesting mechanics from game to game.
Of course, one of the most challenging things a developer can do when creating sequels is to keep the “core experience” of the original games while spicing up the mechanics to keep things fresh. This has gone both well and badly, where in some cases you end up with odd mechanics that seem “out of place” in the series, like vigors in Bioshock Infinite, and sometimes you end up with games that completely overshadow the old ones, such as the newest reboot of the classic “oddworld”. It is up to the developer to study this balance, and to keep the fans of a series happy by safekeeping the parts that the fans enjoy.