Idea

There should be an option to store multiple encryption keys, allowing the user to switch between them for encryption, and the application would attempt each key for decryption.

Reasoning

This would make the encryption feature a lot more useful, especially when using custom keys.

Other Information

• The default key should be hard-coded into the list.
• You should be able to rename keys, so they can be later identified,
• There could be options to assign keys for specific servers, or maybe even for specific users being messaged.
• Caesar might be hard (impossible?) to allow multiple keys for decryption.
  • Probably only allow one Caesar key. It isn't secure anyway.

Instead of trying each and every key (which can take time as the amount of keys increase), it might be possible to store a very short hash in the message itself to identify the key.

There could be options to assign keys for specific servers, or maybe even for specific users being messaged.

I agree with this part, I think the list wouldn't be too useful when it is just arbitrary.
Similarly encryption could be global on one server and PM-only on another.

You can set a key to send message, and you can select specific key by sending with holding some keypress(such as shift).
Select what keys to decrypt.