After checking out mastodon and pleroma docs more carefully I found some functionality that should be added.
- mark conversation as read. Unread conversations should be marked somehow
- remove conversation
- pleroma API allows changing the set of recipients by user
- media can be attached to the message. I suggest just to replace 'Aa' icon in the input by the media uploader.
- as I understand correctly, conversations message is an ordinary status but with direct visibility. I'm not sure that all status interactions are relevant here, but some of them, like status deletion or favorite, can be useful.
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