It would be nice for other plugins to be able to use the API to send messages to discord, I've got a plugin that responds to players in chat about questions they may ask, or whatever the owner sets up, and I would love for it to listen to discord(already possible) and be able to send messages to discord through the api as well.

Use the DiscordUtil class, it has static methods in there for everything related to Discord

Just use the JDA api.