Here is an idea.

Don't restrict character creation. Restrict new accounts to specific servers instead.

I have one character on one server. I wanted to make a new character on the same server but could not because of the restriction. Me having another character on the server is not going to increase the load on it because I obviously can't play both at the same time.

Instead of making Character creation restrictions which are indiscriminate and effect people already on a server, make a restriction that only stops new characters for people who are Not already on that server. This change would not increase the load on the server and people already on it would be able to play with new character designs.