You should start with adding basic commands like deleting mass amounts of messages, checking a users stats like join date, banning and dming them a message. Things that would help with moderation of the discord server.
If I were you I'd familiarize myself with how REST clients function and understand how gateways work. It'll certainly help you out in the long run, but some general features a bot should have would be pruning, chat-filter, and a few commands for discipline perhaps. You should also keep in mind that large HTTP requests (GET, POST, DELETE, etc.) sent through a gateway will be rate limited.