Technology

This already happens right now. If you have 22 open, your firewall is getting hammered with bots trying to get in, regardless of what cipher you're using, trying to exploit known weaknesses.

I know, except they're only ever trying lame user/password pairs that only an idiot would have on their luggage. Same as on asterisk and the bots trying to exploit decades old exploits on wordpress etc. Regardless of whether the site you host is even remotely like wordpress.

I'm not sure how you'd achieve this. If you have a mechanism to change cipher modes then there would be part of the codebase and handshake that validates settings in some way, which adds potential attack vector.

Doesn't need to change the handshake. If the server is mine, and run by me and I decide I was to change say, just the key exchange part of the process. It could be changed without negotiation. I just need to make sure all clients are configured the same way. My point being there wouldn't be a negotiation. If you try to connect to wireguard on my server, you'd need to have the key exchange setup in the same way, with the same parameters too. Yes, it should be entirely optional and require specific configuration changes on both client and server to achieve. So long as server and client are configured with the same parameters there's no negotiation to make. The channel can be setup and if the configuration is wrong it just won't work.