Here's a nice plugin that let's user connect to the server limited by groups. This means that you can set it up that only 20 guests can connect at the same moment, 30 prepayed users, 40 VIP users and unlimited mods and admins. It also queues the users so they have to wait in a line if the server is full for their group and also if they do not try again in less then 2 minutes they will lose their place in the queue. So my suggestion is for someone to update this Open Source plugin and add the functionality for MySQL. The idea is that if the data is stored into MySQL data we can make a .php script that will show the queue list in a web page - atm that's not possible so everybody is in the dark, are they next in the queue or there are 20 more people before them.