Add a feature where people can track specific gameplay statistics or XP. Make a public leaderboard system that tracks these stats of other players so people can actually compete for something. Category examples: Highest level, most cells eaten, most time spent playing each game mode, and best KD ratio (If not too complicated to implement). The leaderboard can track the best 25 players for each category and will be accessible to anyone, through the game. Applying a feature like this will make people compete for something other than just killing other players, which already exists in practically every .io game ever made.