Given that I suspect most use Mastodon for short status updates (under their 500 character limit), it would be nice if there were an option for posting the_content from WordPress’ main body editor along with the URL and/or any hashtags. This way people could post short updates from their blog as status updates/asides and have the full post (or an automatically shortened version if too long) sent over with a link back to the original.
Currently the posting of Title/Link/Hashtags or Title/Content/Link/Hashtags is better for cross-posting longer blog articles which tend to have titles whereas status updates often don’t have titles.
From a UI perspective, it would also optionally be nice to have some type of character counter for the primary text box to fit into Mastodon’s guidelines. I recall there having been a plugin that did just this which might be repurposed: https://wordpress.org/plugins/character-count-for-post-content-excerpt/
To be able to switch between the various modes Title/Link/Hashtags; Title/Content/Link/Hashtags; and potentially this new Body/Link/Hashtags it would be nice if the Mastodon Autopost meta box had the option to also select between them in addition to the “Post to Mastodon” checkbox.