Have more events with cosmetics one time rewards. How about some anime events that Japan got in the past? Make them give anime style cosmetics that you can only get once. That will surely bring people back.
This is the wrong way to go about it, imo. Creating yet more limited-time things when practically EVERYTHING in this game is limited-time already is not going to bring back the people who got burnt out from constant FOMO. Give people the ability to earn what they missed out on, and don't hold them to a timer to get it done. Especially when those limited time things are stuff like Rivalate keys, etc... Stuff that you NEED to perform your best? The fact that they're so limited in quantity, and locked behind limited time events is bullshit. People shouldn't be punished for going on hiatus like that- it only discourages them from sticking around if they do revisit the game.