I am currently making a big game, to combat cheating using the javascript console — i have it so all the players data is server-side and they have to request the data they want to change. Heres the problem what do i name each players variable?, well i could just make it their username?, but it is very easy to change your username, and then you would lose all ur game data 
So my solution would be allowing multiplayer to fetch the player UUID (cuz you guys must have some sort of UUID)
would look like this
CurrentPlayerUUID()
PlayerUUID(player_id)
PlayerUUIDFromUsername(username) // this would also allow to fetch players UUID' that arent currently ingame