After having not played PSO2 in several years, I came back to try out New Genesis, and I'm shocked that this feature still doesn't exist. I can't think of any other MMORPG I've played in years that hasn't had this.
Currently, if a ship is full, you just get an error message that the ship is full and you get kicked out. This is incredibly frustrating for players because you just have to sit on the login screen and hammer the login button, hoping that you'll get in, but having no idea how long it will take.
When you try to log in to a ship, if the ship is full, it should put you into a queue, tell you how many other people are in front of you, and give you an estimate of how long it will be until you're logged in, based on the average rate that other people are logging in & out. When it's your turn, you get logged in.
Aside from being less frustrating for players, this will result in lighter network traffic for busy servers, since they can notify clients when it's their turn instead of having to constantly respond to a crowd of impatient players.