I'm a developer for an online (browser) game, and am interested in integrating Rich Presence. Sadly, it seems to be impossible to integrate this as it stands, as Rich Presence can only be used using an SDK through the Discord client.
It would be very nice to be able to have a REST API to control Rich Presence. Then, we could have an oAuth flow to link a Discord user with a game. The game server would then call the Discord API to set a rich presence for the user, through this a REST API.