system
A chat on subreddit /r/wallstreetbets
human
That's my thoughts as well. Google's had so many failures with messaging.  Google Talk, Hangouts, Duo, Meet, Allo, etc. They pay all these engineers a shit ton of money and can't put together a cohesive messaging platform that integrates well with all their other services. Slack acq would make total sense.

gpt
slack also already has api for google docs integration too so honestly I think this makes sense. 
  
 But Slack+Google may not come in the form of a Google offer buy out, rather it may come in the form of Slack selling themselves to Google because Google would rather do it in house unless they can get slack for cheaper