There will be no official support for restricting a server to a "paid" CA.
Yet still, you CAN set up your own certificate authority, and take money for a certificate. You can also set up a server that only accepts players from that CA.
What does NOT exist in my player auth system, and also will not come, is expiring accounts. Any account, once signed, is valid forever. You could use rotating CAs, and remove the old CAs from a server at the end of the year, and then make a "2009 account", a "2010 account", etc., but what about someone who joins in December? He'd only get an account valid for 1 month.
Also, I do not want to add someone else's plugin for such authentication, but use my own code (I actually majored in cryptography, and even if my own code has serious cryptographic flaws, it just takes someone to point them out and I will understand and be able to fix it). Note that we can NOT use a pre-made SSL library like OpenSSL or GnuTLS, as that also encrypts and does not just sign and authenticate - because if we used one, we would be bound by cryptography export and import restrictions. In some countries there is jail time on the mere import (download) of encryption software, and I don't want anyone to end up in jail for downloading Xonotic. Auth-only would bypass these laws as the government still can read
Also note that cryptographic security for the accounts is absolutely NECESSARY once the accounts are paid for. Otherwise, people can and WILL (I actually know someone who would do it if he could) host hostile servers that will steal your identity, and these people then would abuse it - and if the accounts are worth money, they may even resell the stolen accounts. Not in my neighborhood.
If that system supported Xonotic's way of handling CAs, that would be fine, but it sure will not (e.g. because of lack of expiration date support).
If anything, we can do special accounts that are paid for by a symbolic amount of $0.01 - but actually, we will rather connect this to the forum ID system in a somewhat sensible way, and restrict abuse by certain not yet to be disclosed but somewhat obvious means.