The short version, just make character creation and transfers more intuitive. I'm not saying to let people transfer or create anywhere they want, not by any means. However, if someone is already on a server, then yes. Let them.

A player can only be logged into one character at a time. Whether they only have one character on that server, or if they have the full 8 is irrelevant. The active player count can only increase or decrease by 1, based on whether that player is logged in or not. If someone has active characters (maybe no longer new leaf status, level 30+, whatever action you decide would count it as active), then that server should be opened to them for character creation/transfers.

It's just frustrating, being restricted for seemingly no rhyme or reason. A simple change like so wouldn't solve everyone's problem, but it will help some of us at the very least.