I'm fairly opinionated on that point. I think they should be threaded. I've been stewing on my detest of bolt-on DM systems for years.

If our threads aren't good enough, they should be made better - improving both DMs and threads alike. That assumes the deficit isn't fundamental to DMs being ordered as trees though. But, the average DM UI are lists of messages and lists are trees with only a trunk/branch so we can probably just improve how we display trees in certain situations and get the best of both worlds.

I might add: consider making the UI for DMs unthreaded.  Under a threaded UI, long chains of replies are hard to read because of the max depth.  This makes sense in a free-for-all discussion, but not so much for a 2-person text chain.
  

SimpleStacker

how i'm imagining our MVP of DMs will work

Stacker Saloon

saloon

how i'm imagining our MVP of DMs will work
- unencrypted to start, and clearly labelled as such
- a DM is just an item (like posts and comments) under the hood 
    - like a comment, a message is text field for SN flavored markdown
    - the first DM between sender-receiver is the root of a private SN thread between sender-receiver
    - receiver and sender can reply to each other indefinitely in this private thread
    - no one but sender and receiver can view the thread
        - future versions might allow inviting more stackers to it
        - future versions will be encrypted
    - why: we get to reuse ALL our existing code for threads, live comments, notifications, zaps, etc, and everyone is already familiar with the UI
- not much new UI except 
    - a button to send someone a DM by visiting their profile
    - decoration to communicate the thread is private
    - settings for DM cost to >=1 sat (30% goes to rewards pool, 70% to the receiver)
	   - when stackers receive DMs, the notification has different copy that maybe stands out a little more