Hope you don't mind me coming in here, but could it be that the crystal that would be their ticket home actually has some noticeable difference to it, that can show up at any point on the crystal, which would allow a player to let their character to own such a crystal, but also choose whether or not they get it either?

Ultimately it will still need to be filled for it to work, and maybe it only shows once it's filled (or sometimes it's faulty and sends people back when it shows up randomly and there's the reason people go home who wouldn't want to go home), but allows people to play with this part of the game if it works for them than to have to say that their crystals are taking five hundred years to complete.