Must take into account character limitations of target service.
Should include handle lookup during post creation for things like @\someone.
Hashtag autocompletion?
Handle scheduled reposts (retweets, boosts) so that a user doesn't overwhelm their followers' feeds.
Have some sort of default queue with slots for casual scheduling when needed.
Ability to cancel, edit, or reschedule a queued status. NOTE: For mastodon this means a remote call since scheduled posts live on their destination server.
@\someone.