I have a couple of suggestions that, in my opinion, would make Slack even more awesome than it already is.
1) I sometimes find that a conversation or channel has been cluttered up with a side conversation (often of my own making) that would have been better to break out into a thread. I would love a feature that would…
a) allow you to select specific messages in a single conversation or channel and propose that they be moved into a thread,
b) request permission from the message owners, and
c) move the messages into the thread
2) Similarly, a feature that would allow you to request that another user's large message be converted into a collapsible snippet.
I feel like these two features would be a nice aid in keeping our conversations & channels focused.